Issue 493: Introduce new naming convention for class files

Resolves #493 

This renames all the class files and puts them in a classes directory. It also updates the language files.

@shadyvb @fjarrett Please review.

@westonruter - could you please let me know what you had in mind in terms of [autoloading](https://github.com/x-team/wp-stream/issues/493#issuecomment-41989903)?

Please do not merge yet, I'd like to hear from Weston first.