[Fix] Add missing audiochannels rule to PlayerSelectionRule.cpp

This adds the missing test against the audiochannels rule in PlayerSelectionRule.cpp.  Fixes http://trac.xbmc.org/ticket/13536

I wasn't entirely sure what the best way to convert an integer to a string was, so I just copied this code from CSFTPSessionManager::CreateSession (which was about the only place I could find that already did it).