JRuby support

This commit has no build smarts. Here's the process:

```
$ javac -cp ../jruby/lib/jruby.jar -d ext/java ext/java/com/headius/jruby/fast_blank/FastBlankLibrary.java

$ jar cvf lib/fast_blank.jar -C ext/java/ com

$ jruby -Ilib -e 'require "fast_blank"; p "".blank?'
true
```

Proper support will use `rake-compiler` to build the jar.

ext/java/com/headius/jruby/fast_blank/FastBlankLibrary.java

@@ -0,0 +1,108 @@
+package com.headius.jruby.fast_blank;
+
+import org.jcodings.Encoding;
+import org.jruby.Ruby;
+import org.jruby.RubyString;
+import org.jruby.anno.JRubyMethod;
+import org.jruby.runtime.ThreadContext;
+import org.jruby.runtime.builtin.IRubyObject;
+import org.jruby.runtime.load.Library;
+import org.jruby.util.ByteList;
+import org.jruby.util.io.EncodingUtils;
+
+public class FastBlankLibrary implements Library {
+    public void load(Ruby runtime, boolean wrap) {
+        runtime.getString().defineAnnotatedMethods(FastBlankLibrary.class);
+    }
+
+    @JRubyMethod(name = "blank_as?")
+    public static IRubyObject blank_as_p(ThreadContext context, IRubyObject self) {
+        Encoding enc;
+        int s, e;
+        byte[] sBytes;
+
+        Ruby runtime = context.runtime;
+
+        RubyString str = (RubyString) self;
+        enc = str.getEncoding();
+        ByteList sByteList = str.getByteList();
+        sBytes = sByteList.unsafeBytes();
+        s = sByteList.begin();
+        if (str.size() == 0) return context.tru;
+
+        e = s + sByteList.realSize();
+        int[] n = {0};
+        while (s < e) {
+            int cc = EncodingUtils.encCodepointLength(runtime, sBytes, s, e, n, enc);
+
+            switch (cc) {
+                case 9:
+                case 0xa:
+                case 0xb:
+                case 0xc:
+                case 0xd:
+                case 0x20:
+                case 0x85:
+                case 0xa0:
+                case 0x1680:
+                case 0x2000:
+                case 0x2001:
+                case 0x2002:
+                case 0x2003:
+                case 0x2004:
+                case 0x2005:
+                case 0x2006:
+                case 0x2007:
+                case 0x2008:
+                case 0x2009:
+                case 0x200a:
+                case 0x2028:
+                case 0x2029:
+                case 0x202f:
+                case 0x205f:
+                case 0x3000:
+                    /* found */
+                    break;
+                default:
+                    return context.fals;
+            }
+            s += n[0];
+        }
+        return context.tru;
+    }
+
+    @JRubyMethod(name = "blank?")
+    public static IRubyObject blank_p(ThreadContext context, IRubyObject self) {
+        Encoding enc;
+        int s, e;
+        byte[] sBytes;
+
+        Ruby runtime = context.runtime;
+
+        RubyString str = (RubyString) self;
+
+        enc = str.getEncoding();
+        ByteList sByteList = str.getByteList();
+        sBytes = sByteList.unsafeBytes();
+        s = sByteList.begin();
+
+        if (str.size() == 0) return context.tru;
+
+        e = s + sByteList.realSize();
+        int[] n = {0};
+        while (s < e) {
+            int cc = EncodingUtils.encCodepointLength(runtime, sBytes, s, e, n, enc);
+
+            if (!isSpace(cc) && cc != 0) return context.fals;
+            s += n[0];
+        }
+
+        return context.tru;
+    }
+
+    // MRI: rb_isspace
+    private static boolean isSpace(int codepoint) {
+        long c = codepoint & 0xFFFFFFFF;
+        return c == ' ' || ('\t' <= c && c <= '\r');
+    }
+}

lib/fast_blank.rb

@@ -0,0 +1,7 @@
+case RUBY_ENGINE
+  when 'jruby'
+    require 'fast_blank.jar'
+    JRuby::Util.load_ext("com.headius.jruby.fast_blank.FastBlankLibrary")
+  else
+    require 'fast_blank.so'
+end