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
Add Actress::AdHoc actor

it's behavior is given by proc passed on initialization
Configurable Core

executor can be configured, other options will follow
spawn can be also called with hash instead of class, name, *args
configurable log level
Terminate on error in message processing

since we do not know ho to restart yet
minor updates in logging and comments
Add basic spec
Replace Array of children with more effective Set
Make name and executor publicly accessible
Fix attar accessibility of @parent_core and @logger
AdHoc calls message processing block with instance_exec and it accepts *args