Use istanbul/lib/cli.js instead of node_modules/.bin/istanbul

Due to the way, "bin"-files are distributed into the node_modules/.bin
directory on Windows, the task "test:cov" did not work on Windows.
This commit uses the node-script directly.

tasks/test.js

@@ -41,7 +41,7 @@ module.exports = function(grunt) {
   grunt.registerTask('test:cov', function() {
     var done = this.async();
 
-    var runner = childProcess.fork('node_modules/.bin/istanbul', ['cover', '--', './spec/env/runner.js'], {stdio: 'inherit'});
+    var runner = childProcess.fork('node_modules/istanbul/lib/cli.js', ['cover', '--source-map', '--', './spec/env/runner.js'], {stdio: 'inherit'});
     runner.on('close', function(code) {
       if (code != 0) {
         grunt.fatal(code + ' tests failed');