Merge pull request #6 from seadowg/master

Allow multiple callback regsitering
Fix bug with select

select was returning true/false rather than the event itself. whoops
another bugfix, on events not handled for stream derivative
Fix bugs with Stream derivative #on_value

The Map and Select Streams were not respecting the change
from an @on_value callback to a @callbacks list (my bad).
I've moved this logic into a protected #occur method on Stream
to make extending Stream cleaner.
Memoize callbacks array