Merge pull request #184 from celluloid/self-leak-in-supervisor-group

Do not leak self in SupervisionGroup#initialize
Rename InvokeBlock to BlockCall

Add a syntax for declaring that blocks run on the receiver

revert whitespace change

Remove dangerous execution of blocks

Move BlockProxy creation into Call

Cleanup #1

Cleanup #2

Actor.call Task is suspended until the final response is returned
Use the BlockProxy in the BlockCall