added not_ methods to find objects not in the given state

aasm has built-in methods to find objects in the specified state; for example, for state 'foo' there is SomeModel.foo  will return a set of records with that state.

I've added similar methods prefixed with not_  , to find objects that are in any state other than the one specified.  For example,  SomeModel.not_foo would return a set of records with state 'bar' or 'woot' but not 'foo'.