Introduce Poise stat

Closes #80

I went with a percentage chance here as opposed to a damage threshold
because I think it's easier to scale when balancing combat numbers. We can
always change the formula to a damage threshold later if that's the desired
method.

I decided to keep the hit sound effect when damage recoil is skipped, since
it's the only form of feedback when combat text is off.
Merge pull request #322 from dorkster/bugfixes

Introduce Poise stat