Add support for extra configs for uWSGI server

I added support for passing extra configs to the uWSGI server configuration. I needed this personally in order to control socket permissions (per [this](http://uwsgi-docs.readthedocs.org/en/latest/tutorials/Django_and_nginx.html#if-that-doesn-t-work)), but could see it being useful in general. Here are specific changes.

* Add config param to uwsgi class
* Update uwsgi template to handle extra config
* Update README to reflect uwsgi config changes
* Misc README cleanups