Task queue

Adds a worker queue service based on Python-RQ, and uses it to handle securely removing files. This process can be very time-consuming, especially with large files or lots of files. Currently, the process blocks the response the web server, causing the document interface to hang for a *long* time (until the srm is done, which can be many minutes for very large files).

This task queue could also be used to generate GPG keys, which would have several beneficial properties. That is not implemented in this PR but this PR is a prerequisite for that.

Fixes #562 

I'd like reviews from anybody, but especially @dolanjs on fef0f31! @jacksingleton and @charltonaustin might also be interested in this one.