[fix] Handle vows for events that execute multiple times (fixes #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.
Minor style fix.