better performance for typecast_attribute

https://gist.github.com/4551282

is a bit more clearer, than https://github.com/cgriego/active_attr/pull/110
(same map, just not recreated on every method call)
Merge branch 'master' of git://github.com/mindreframer/active_attr into mindreframer-master
Freeze TYPECASTER_MAP and mark private to discourage dependence on an unpublished API