Updated requests package to v2.2.1

It was included, but never used. However, lots of code can improve from this.
Replaced urllib with requests, part 1
Moved request method to helpers. Removed unused imports
Replaced urllib2 in version check
Bugfixes
Replaced urllib2 in albumart and cache
Replaces urllib2 in importer, lastfm and lyrics
Replaced urllib2 in mb, notifiers, postprocessor, searcher and transmission
Refactored few names
Fix trailing spaces
Deffer searches until after all albums have been found
Bugfixes
Added six v1.6.1. Is required for some (newer) modules, like html5lib
Return empty list instead of None. It's not necessary to only subset the list if there are items
Refactored Last.FM code
Moved search to the end to defer it more
Moved all request methods to seperate file, since it is getting a core function
Restored logger functionality, since requests are seperated now
Prefer to log debug when it is not white listed
Bugfixes