Hipchat multi event

Add support in the hipchat output for multiple events.

The `rollup` function will send a vector of events to its children, if using hipchat in conjunction with `rollup` you'll get a single event posted to hipchat with no information on it.

This PR allows hipchat to accept vectors of events (code based on riemann.common/body).

I've added tests, but they will only work with a hipchat account. If you run them without these changes you'll see the multi event failing in the chat.

@aphyr - I've setup a trail hipchat account, if you want to test the changes but don't have an account I'm happy to send over the details.