add Promise.cast
Add Promise#finally
Merge pull request #131 from stefanpenner/finally

add support for Promise.cast + Promise#finally
add Promise.cast
Add Promise#finally
perf boosts

- custom pub-sub for propogate state changes instead of event emitter
- minimize closure allocations.
- as we only need to publish once, cleanup subscribers after publish
fix all to work if an array element is null
tidy-up RSVP.hash
fix test, that where using non-public and no
longer existent API's