moving _boom.py to boom.py, adjusting other code as necessary

Was thinking about doing this anyway, and then I saw [Issue #31](https://github.com/tarekziade/boom/issues/31).

``python setup.py test`` all tests pass most of the time. The dns resolve test fails occasionally, but I think that's a DNS issue on my machine, not an issue with the code, since it usually passes.

Not sure how relevant this is, but when I was trying to work on this, I was using ipython to test importing stuff and it was throwing a weird exception. It turns out that this is because ipython and gevent don't play well together, and had nothing to do with changes I was making.