Starting implementation of stateful machine.
Adding missing methods/properties to complete stateful entities.

Missing documentation yet.
Applying changes on test_stateful as suggested by @nicoddemus

Tks for the tip! ;)
Updating documentation about stateful entities.
Fixing Repo.state to return last_build_state reference.

Also added a line to test state against last_build_state so coverage may be improved.
Merge pull request #3 from menegazzo/state-machine-entities

State machine entities
Close #1