Implemented Pull2Refresh in the KeyListFragment

Added KeyUpdateHelper and extended ProviderHelper for better fingerprint collection
Added a class for handling the list in the swipe2refresh layout that prevents scrolling bugs
Added Pull2Refresh to the KeyListFragment
Removed unnecessary querying of keyservers, reimplemented the ListAwareSwipeRefreshLayout and implemented a pull-lock in case there are no keyservers
Merge pull request #843 from da-kid/master

Implemented Pull2Refresh