Actress merge

continuation of #72, needs more work and discussions
- [x] change Abstract class to module
- [x] remove Algebrick dependency
- [x] remove Atomic dependency
- [x] cleanup
- [x] split-files
- [x] tests
- [x] it deadlocks in rspec environment, 'no live threads'
- [x] check all TODOs left in the code
- [x] documentation
- [ ] fix rbx and jruby-head test ?
- [ ] **merge**
- [ ] better tests
- [ ] better documentation
- [ ] add example of using Actors with Algebrick and Hamster
- [ ] basic supervision and restarts
- [ ] actor state monitoring as in Akka

**Later**
- [ ] un/become ?
- [ ] dead letter routing ?