Library concurency refactoring

Hi, developers!

I need to get your code review.

Here is my branch *"LibraryConcurencyRefactoring"* developed as Google Summer of Code 2013 project.

Here you can read about our implemented ideas: http://mixxx.org/wiki/doku.php/lambda_scheme.

You can contact me via GitHub -- https://github.com/troyane, e-mail -- troyan3 at gmail dot com, or at mixxx irc channel -- tro.

P. S. There are some known issues with Rhythmbox playlist. It will be solved soon.