Fixing error when using YuiCompressor due to 'abstract' being a reserved...

Fixing error when using YuiCompressor due to 'abstract' being a reserved word in js. Anytime  `abstract` is used it should be enclosed in quotes as this is a reserved word for future implementation in EcmaScript, much like `class`. It may be best to use another word, say `$abstract` to prevent such possible issues.