Allow play to work from a non-web root location. Example: http://<host>/play/

So part of the idea of me running play through apache was to be able to run play as a sub directory on my server. This doesn't work as play is written currently. This fixes that problem, probably not the correct or most efficient way, but it is an approach to a solution.

- If you want to run play from the web host root, you don't need to do anything.
- If you want to run play from a web host non-root path, example `/play`, then you just stick /play in `config/play.yml` for `uri_prefix: ` and you are off and running on the subdirectory.

I could not find any documentation (or much of any documentation for it) of how to wrap sprocket maps with a uri prefix, but that would be an awesome feature. If you could do this, then you could just request assets with assets['file'] and not have to define the full static uri in the templates.

Not expecting this to be merged as it sits as this doesn't seem to be the most logical solution to the problem. It is just bringing an idea to the table and attaching some basic code.