Merge pull request #86 from pitr-ch/minor

Convenience methods for Obligation and add_observer returning self
Raw port of Actress

needs more work and discussions
Change Abstract actor class to a ActorContext module
Remove Algebrick dependency
when spawn returns children are set
Reject all envelops sent to termiated actor before actor is GCed
Remove dead code
Remove dependency on Atomic
Remove unused mutex

no instance variables are being reassigned
Add Reference#ask!
TODOs and comments update
Split files and add documentation