jquery.client unit testing

jquery.client unit testing
- More elaborate caching in the plugin in order for the code to be testable
- Added support for passing custom user agents (although the default behavior remains unchanged)
- Added support for passing custom profile-objects to $.client.test
- Added test case for all supported browsers according to http://www.mediawiki.org/wiki/Compatibility#Browser
- Replaced userAgent.toLowerCase() with userAgent since this string was already converted to lower case a few lines up

This will likely prevent bugs like bug 27652 and bug 29446.