Stabby proc and attr init

Yang, this branch contains a merged version of both previous pull requests. Specs should explain pretty well. In the future I'll probably open issues first to discuss, but if this code works for you, please feel free to take it.. If it needs more love - let me know ! 

PS - I've submitted a talk for RailsConf on reactive programming-  I'll know in a week if they accept-  yours is the library I intend to base the talk on - thus my increased interest of late for contributing :)