Move Statesman::Transition into the Adapters namespace

It's only used specifically with the Memory adapter anyway.
Remove transition_class config option
Add Statesman::Adapters::ActiveRecordTransition
Update generator to include transition mixin
Update readme
Merge branch 'master' into transition-module

Conflicts:
	lib/statesman.rb
Merge branch 'master' into transition-module
Initial MongoidTransition module
Merge pull request #7 from gocardless/transition-module

Transition module