Implement Behavior.execute().

This required adding to BehaviorManager the ability to get a list
of the entities which have a given Behavior enabled.
Moved an if statement outside of a for loop.
Fixed typo in BehaviorManager comment.
Removed erroneous word.
Merge pull request #9 from vorporeal/base-devel

Implement Behavior.execute().
Merge branch 'master' into canvas