Added new functionality.

* Add support for artwork
* Simplified partial responses
Updated gitignore for Cython
Updated tests
Typo in gitignore
Refactoring of the code.

* Easier-to-use object revisioning with better support for custom backends
* Speed improvements by removing a lot of 'safety' checks from the DAAP code.
  While one may argue this isn't a good idea: when creating 50.000+ objects,
  you will know when it crashes.
* Support for iterating DAAP structures. Saves a lot of memory
* Added generic utilities
* Optional Cython augmentation for more speed and memory improvements
* Renewed example