Add Flagging.with_flag(reason), as per README

Hi! 

I was trying to use the ``.with_flag(reason)`` scope for Flagging, as it is documented in ``README``, but I realized there's no scope for that in the code. I added it 

Also, I fixed the spec ``schema.rb`` to include ``flaggings_count`` column (all tests were failing before that). 

Hope you like to include this!

Regards, 

PS: I also noticed generator spec is red in Rails >= 4.0, but I don't know how to fix that. You can have a peep here: https://travis-ci.org/dgilperez/make_flaggable/builds/35460558