Add support for abort if pending

[hmadison]

Resolves #7

cc @metaskills

[metaskills]

Nice! I have often considered the abort_if_pending_migrations kind of private that other's call. Would the test still work if we called test:prepare or maybe even test on the dummy app?

[hmadison]

@metaskills It would, but i'm trying to hook in places that won't cause some kind of astonishment later down the road. While abort_if_pending_migrations may be private, we already dig into some private apis to make this gem work and I rather pick the more logical but private hook if i can.

[metaskills]

Roger roger.