Merge pull request #53 from collectiveidea/call

Use Ruby's idiomatic "call" method rather than "perform"
Remove success/failure convenience methods from the interactor instance
Change Interactor.call to return the context rather than the instance

This also introduces a similar class-level rollback method in order to
allow organizer rollback without needing to internally track and store
instances of called interactors.
Remove setup in favor of before/after hooks

The new hooks are not evaluated until runtime, unlike the old setup
method which was evaluated during initialization.
Refactor Interactor#call_with_hooks

This helps pave the way for "around" hooks
Add a slightly gross implementation of "around" hooks
Use instance_exec for all forms of hooks, rather than instance_eval