cleanup/rework how setups/teardowns are run

Previously the single `setup` method was used both to add blocks
or methods _and_ to run the added setups.  This was originally
done to minimize "method pollution" on the context class, but this
reasoning doesn't seem to outweigh the added complexity of determining
which case this method is being called for.

This breaks out a new method `run_setup` to more implicitly know
when you are adding a setup or running them.

Same thing goes for the teardowns.  This doesn't change the "intended"
API as `run_setup` is only called internally.  This is in prep for
doing the "around" callbacks feature.
Merge pull request #178 from redding/kr-setup-teardown-cleanups

cleanup/rework how setups/teardowns are run