Support addBatch() with object with more than one key

[evanp]

We should be able to do this:

vows.describe('a test suite')
  .addBatch({
      'When something happens': { // ...
      },
      'When something else happens': { // ...
      }
   })
   .export(module)

I'm not sure how this actually shakes out. I think the easiest thing to do is just add multiple Batch objects, although that makes the method signature addBatch() (singular) a little misleading.