WIP: Dynamic proxy for IPython, other such plugins.

TODO (PREMERGE):
- Replace file operations with sqlite database + write file lock (for multi-process Galaxies).
- Document configuration.
- SSL support (pretty easy to add - though I would expect most people to just use a nginx/apache to do this).

TODO (POSTMERGE):
- Add alternative docker version.
- Update galaxy-ipython to support something like proxy-prefix - configure along with the rest of this.