[fix] Handle vows for events that execute multiple times

Fix for issue described in #191.

Add failing test for events that fire multiple times.

In `lib/vows/suite.js` add a property `count` to each vow which is incremented every time the vow is executed.

In `runTest` (in `addVow` in `lib/vows.js`), on each execution increment `batch.total` if a vow has been executed more than once.