Adding support for a callback to jquery.byteLimit

Adding support for a callback to jquery.byteLimit
* Fixes (bug 29455) Add support for a filter callback function in jQuery byteLimit plugin.
* Adding unit tests for it
* Changing if-statements in mw.Title's helper functions for regular expression matches. It should check wether the value is not null or undefined. Before it was just a plain if-statement which meant that an empty string would also return false, which made the new byteLimit's tests fail (mw.Title.getMain() expects _name to be a valid string when it does ucFirst() and substr() etc.)