Allow a block to be called when running a transition

[Intrepidd]

Hi,

This is based on the former run_transition method of the old acts_as_state_machine gem from @rubyist

Sometimes, you want to do an action only if the transition succeeds, but you also want to call the failed callbacks when it does not.

I did not find any way to do this natively (feel free to point out some existing ways if there are), so I implemented this little feature.

object.transition! do
  some_code
end

some_code will be called only if the transition succeeds, before launching the after_enter callbacks.

[coveralls]

Coverage increased (+3.1%) when pulling 46878f9 on Intrepidd:master into 8a14cac on aasm:master.

[Intrepidd]

Hey ! Any news ?

[alto]

Sorry for the delay. I just saw the merge conflict and put it back without taking a closer look. At the end, it was pretty easy. Thanks a lot for the nice change!

Do you mind adding some lines to the readme? I'm happy to release a new version then. Let me know if you want to update the readme. Otherwise I'll come up with information about the new possibility.

[Intrepidd]

No problem !

I'll issue a new PR shortly with README changes :)

[Intrepidd]

See #97 !