First pass at JRuby support.

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

```
$ cd ext/java/

$ javac -cp ../../../jruby/lib/jruby.jar fast_blank.java

$ jar cvf ../../lib/fast_blank.jar fast_blank.class
added manifest
adding: fast_blank.class(in = 2182) (out= 1121)(deflated 48%)

$ cd ../..

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

ext/java/fast_blank.java

@@ -0,0 +1,100 @@
+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.util.ByteList;
+import org.jruby.util.io.EncodingUtils;
+
+public class fast_blank {
+    @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:
+                    case 0x180e:
+                          /* 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 (!Character.isSpaceChar(cc) && cc != 0) return context.fals;
+            s += n[0];
+        }
+
+        return context.tru;
+    }
+
+    public static void init(Ruby runtime) {
+        runtime.getString().defineAnnotatedMethods(fast_blank.class);
+    }
+}

lib/fast_blank.rb

@@ -0,0 +1,7 @@
+case RUBY_ENGINE
+  when 'jruby'
+    require 'fast_blank.jar'
+    Java::fast_blank.init(JRuby.runtime)
+  else
+    require 'fast_blank.so'
+end