Added a few more tests. Improved from_dict.

* from_dict will now apply any data without concern for what fields
  are defined which I believe should be desired behavior. This
  completes part of #8, still need to ensure validate handles this
  case.
Added to_dict test and improved implementation.

* Undefined fields are now returned as well.
Improved API documentation a little bit.

* Added cross-references where appropriate.
* Added the metclass to the API documentation.