Updated the basic repository files. Structured API documentation, added ...

I 'modernized' the basic repository files a bit. Furthermore, I added the `CONTRIBUTING.md` file. Users will be hinted to this file when the submit a pull request or post an issue. See https://github.com/blog/1184-contributing-guidelines for more information.

The `CONTRIBUTING.md` is currently what I could come up with, and serves as a start. For the development part, I choose PEP8 convention for future contributed code, which is what I try to follow in my pull requests, and what is suggested in #1801. In my opinion, Headphones needs one to stay readable and maintainable.