Add Redis Cache

I added support for caching backed by redis. When running the tests, it assumes that Redis is running on localhost:6379, which is the default. It also supports connecting via a unix socket by setting the host to ``file:///path/to/socket``.

I can update the docs after I get some feedback, and also after I see if this is something that would be useful within the revel framework. This is my first contribution to a go project, so please let me know what can be done better and I can update it :smiley:.