Added call_in_(twisted|green)_thread utility functions

Ignore-this: fb8194631733493be9dfad9f7104ad0d

darcs-hash:20100104164750-1e3d8-393e731fdb6c8524a0c58b5d6445fd0a27dda31b
Added limit utility function

Ignore-this: 5d55daf08545e673562d50841faf5c81

darcs-hash:20100104164825-1e3d8-c77eb6c3a348184db5e080a30aaed75c1f039ec9