Refactor away the Feature object

[young-steveo]

After a few rewrites of the code, the Feature class has become superfluous.

How it works now:

1. Feature\Builder accepts and manages Behavior objects.
2. When Feature\Builder::execute is called, it attaches the correct Behavior to a new Feature object, then calls Feature::execute.
3. Feature::execute simply delegates to Behavior::execute.

Since the Feature class doesn't really do anything else, it stands to reason that Feature\Builder::execute could just delegate to the correct Behavior::execute method directly, rather than using Feature as a proxy.