Automatic Slicer downloading

Basically, I've created a repository at [botqueue-slicers](https://github.com/Jnesselr/botqueue-slicers) where each branch is a slice engine for a particular host and slicer version.

This pull request contains the code to automatically download and install a slicer version based on the repo I linked above. It also removes any code from the slicers folder through gitignore. That way, we don't have to pull larger and larger repos down with all of the slicer code. I'd like to purge them from the repo, but that's a decision that we'll have to work on together.

I've tested this with downloading slic3r 0.9.9, 0.9.10b, and 1.0.0RC1 all at the same time on the raspberry pi.

I still need to make these changes:
- [x] Switch to urllib2 instead of urllib
- [ ] Give an error to BotQueue to display as a slicing failure

I'm also working on making this so it will work with slicers as well.