Add docs/test showing single filepath

I found it difficult to understand how to serve a single static file using Klein. dreid and Hawkhowl told me how this could be done. I thought it would be helpful is this solution might be accessible to others.

The patch adds 
 - a test for klein serving a single file
 - docs explaining how a user should serve a single file.

Lastly - dreid had suggested that it might be helpful if we were to teach Klein about FilePaths. Here is his comment from IRC:

> It might be worth it to teach klein about FilePath specifically instead of just resources, then it could distinguish when a File object (which subclasses FilePath for reasons) is a File as opposed to a directory and do the right thing.