Merge pull request #7 from clintongormley/master

Clear up resolved and rejected callbacks on resolution or rejection
Merge pull request #9 from clintongormley/master

Allow then() to execute in the context of the event loop.
Updating some docs
adding some docs
throwing in an AnyEvent and EV backend
some tests
make sure to fire all callbacks as a complete set
notes in the Changes file
Added then_discard() to allow the promise chain to be broken
This is useful for keeping the return stack flat while using
promises recursively.

Also added tests and Promises::Cookbook::Recursion

Fixes #10