refactor acts_as_asset
refactor acts_as_asset

remove some unneeded things, since rails handles them for us. this
should not change the external behaviour. and some tests, i implemented
to check this, still pass. (these tests will be comitted, soon)

RAllner/fassets_core,fassets/fassets_presentations,fassets/fassets_presentations,fassets/fassets_resources,fassets/fassets_resources,RAllner/fassets_core