Added initial assignments code

Hey Remy,

You may recall that we chatted via email a few days back about adding some functionality to make it easier to use jsbin in a school setting. I've taken the first small step along that road with this code. It does the following:

- Adds an assignments table which points to a url/revision in sandbox. That url/revision is the initial template for an assignment.
- Adds a /assign/:number route. Students would use that (via a yet to be created dashboard) to start working on assignment :number. That URL clones the html, css, and javascript for the given assignment.

I added all of this functionality to the sqlite backend, but have neglected the others. If you think I'm on the right track here and you're interested in merging I can update those as well.

Thanks,
Oliver