Test cases for Document

Added a module for unit tests targeting Document.  

I mentioned in the stand-up this morning that I think many of the features (new and previously existing in 2.4.0) of mongo's Document don't have a place in mungedb-aggregate since we simply use objects instead of a Document construct.  However, I wasn't too sure about the following:
 - (1) the C++ macros for DOC and DOC_ARRAY
 - (2) MutableDocument/MutableValue
 - (3) serialization/deserialization for Sorter.

If anyone thinks those should be implemented, let me know.