Makes mocha.js YUI-compressible

Before this diff, trying to compress mocha.js with YUI compressor would result in the following output
```
=> java -jar yuicompressor-2.4.8.jar mocha.js
[ERROR] in mocha.js
  1733:19:missing name after . operator
[ERROR] in mocha.js
  1734:16:identifier is a reserved word
[ERROR] in mocha.js
  1735:9:missing ; before statement
[ERROR] in mocha.js
  1736:17:identifier is a reserved word
[ERROR] in mocha.js
  1738:2:missing ) after argument list
[ERROR] in mocha.js
  1787:15:missing ( before function parameters.
[ERROR] in mocha.js
  1787:15:missing } after function body
[ERROR] in mocha.js
  1788:22:missing ; before statement
[ERROR] in mocha.js
  1789:22:invalid return
[ERROR] in mocha.js
  1790:22:invalid return
[ERROR] in mocha.js
  1791:22:invalid return
[ERROR] in mocha.js
  1792:9:invalid return
[ERROR] in mocha.js
  1793:1:syntax error
[ERROR] in mocha.js
  1803:14:identifier is a reserved word
[ERROR] in mocha.js
  1809:1:syntax error
[ERROR] in mocha.js
  1815:30:missing ; before statement
[ERROR] in mocha.js
  1816:21:missing ; before statement
[ERROR] in mocha.js
  1817:27:invalid return
[ERROR] in mocha.js
  1818:9:invalid return
[ERROR] in mocha.js
  1819:1:syntax error
[ERROR] in mocha.js
  1821:2:syntax error
[ERROR] in mocha.js
  5600:1:syntax error
[ERROR] in mocha.js
  1:0:Compilation produced 22 syntax errors.
org.mozilla.javascript.EvaluatorException: Compilation produced 22 syntax errors.
	at com.yahoo.platform.yui.compressor.YUICompressor$1.runtimeError(YUICompressor.java:172)
	at org.mozilla.javascript.Parser.parse(Parser.java:396)
	at org.mozilla.javascript.Parser.parse(Parser.java:340)
	at com.yahoo.platform.yui.compressor.JavaScriptCompressor.parse(JavaScriptCompressor.java:315)
	at com.yahoo.platform.yui.compressor.JavaScriptCompressor.<init>(JavaScriptCompressor.java:536)
	at com.yahoo.platform.yui.compressor.YUICompressor.main(YUICompressor.java:147)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at com.yahoo.platform.yui.compressor.Bootstrap.main(Bootstrap.java:21)
```
After this change, mocha.js compresses as you would expect, without errors.

The reason why YUI failed to compressed: use of `long` and `short. They are reserved word in ECMAScript since [ECMAScript v2](http://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%202nd%20edition,%20August%201998.pdf):
![reserved_words](https://f.cloud.github.com/assets/208469/1518024/00f25afe-4b2e-11e3-834e-64920fd54289.png)

Cheers,
Arnaud.