Filter for log entries and style improvements

Using the log output from debug plugin, it can be a mess if we have a lot of log entries. So I have added the option to filter by priority and category. Added some style improvements, as you can see on the attached images on the track item.

Track item: #32936 - http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32936&start=0