CC-5555 : Implement simple caching for preferences

using apc to save/fetch preferences to/from a cache.
CC-5555 : Implement simple caching for preferences

fixing up code so user id is never passed in manually to a preference getter.
CC-5555 : Implement simple caching for preferences

making sure locale/timezone is returned properly in generic calls.
CC-5555 : Implement simple caching for preferences

better to include the key in the error message
CC-5555 : Implement simple caching for preferences

forgot to get rid of user id here.
CC-5555 : Implement simple caching for preferences

cache key creation change.
Merge branch '2.5.x'
Merge branch 'master' of github.com:sourcefabric/Airtime