Added regex filter to ensure callback (jsonp) is valid JavaScript function.

This will overcome an XSS vulnerability that allows any string data
inside the callback parameter. This filters the callback parmeter to verify it only contains alpha (also
capital), numeric characters, underscore, and dollar sign ($), also ensuring the begining is not a number.

I believe this is a very minimal issue. But a simply safe approach.

case: documented on [stackoverflow](http://stackoverflow.com/questions/8750469/apparent-jsonp-xss-vulnerability).

Please note, I do not have periods allowed in this filter. I see how this could become an issue for complex apps. Let me know your take on the period as part of the callback.