Fabric deployment script

I've got a working deploy path with `fabric` that can stop, start, restart the application, and deploy new code on demand. This PR is **downstream** of #47, so merge that one first.

This does a few things, including some fixes along the way. Would love @khandelwal's eyes on the fixes, since I think they relate to his work in #46.

Note that `fabric` requires Python 2.X, and our app requires Python 3.X. I'm managing this by having a `fab` virtualenv with a Python 2.X frozen inside it, and switching to it before running `fab` commands.

* Moves the gunicorn config into a `deploy/` directory, to a (git ignored) `deploy/config.py` file. A `config.py.example` file sits next to it, which can be copied and adapted.
* Adds a `fabfile.py` that can be used to deploy and control the application. Commands:

```bash
fab start         # start stopped app
fab stop          # stop running app
fab restart       # restart running app (will update changed code, will not update changed environment)
fab deploy        # deploy when the app is running
fab deploy_cold   # deploy when the app is stopped
```

* Tore out hair figuring out why fabric wasn't starting `gunicorn` properly, eventually [figured out](http://stackoverflow.com/questions/17748234/unable-to-start-gunicorn-with-fabric#comment26533754_17986215) that fabric requires `pty=False` for [background processes](http://docs.fabfile.org/en/1.7/usage/interactivity.html). (This has never affected me before when using fabric to run `unicorn` processes in Ruby-land.)
* Commented out `ALLOWED_HOSTS` in `settings/base.py` because it was somehow obscuring the `ALLOWED_HOSTS` set in `settings/production.py` from being seen. @khandelwal, any ideas why this was? `production.py` imports from `.base.*` before anything, seems like it should be overwritten.
* Fixed bug in `settings/production.py`, added the missing dot-prefix in `from .local_settings import *`.

Not in source control, but done on our server:

* Created a production `local_settings.py` file, pointing to the server's own Postgres database.
* Created a production gunicorn `config.py` file, configured to launch 2 worker processes.
* Began logging gunicorn access and error log output.
* Cleared database, ran migrations, reloaded data using the latest code and data.

If this is good, I can proceed to adding deploy webhooks, and updating fabric with commands to control them.
All of the functions in here could benefit from some brief documentation (docstrings are fine). For example, it's unclear what the intended difference between deploy() and deploye_cold() is.