Controller filters can shortcut request lifecycle by returning a Response


  * If a before filter returns it, both the action and the after filters will be skipped (as well as any remaining
  * If an after filter returns it, any remaining after filters in the block will be skipped.
  * There is no way for the action result to skip the :after filters.

Closes #21
Signed-off-by: Josep M. Blanquer <blanquer@rightscale.com>
The mixing of `result` and `r` is inconsistent. What if we stuck with `result` as an expected final value, and found a name for intermediate results that may mean we short-circuit evaluation?