Add fabric file.

Here's a totally untested fab file, based on one I made for another project.

Basically if you're doing deployments, you need to copy fab_targets.py.example to fab_targets.py, and edit that to point at your deployment targets. You can then deploy by running something like `fab prod deploy` or `fab stage deploy` in perma_web.

This also has some handy tasks for local devs, such as `fab south_out` and `fab south_in`, which I find easier than remembering the actual south commands. We could add tasks for other dev setup stuff like starting celery.