allow retrieving the current event (see #159 and #168)
version bump to 3.4.0
Merge branch 'master' into aasm4
callback :on_transition renamed to :after and changed its binding (see #59)
Merge branch 'aasm4_squash_merging_stiff_master' into merging-stiff-master

Conflicts:
	lib/aasm/transition.rb
	spec/unit/transition_spec.rb