All connection-related functions are now in connection.py.
Created a ConnectionManager class for interacting with a collection
in the database. Top-level document classes have an instance of
a ConnectionManager (Document.collection).
Defined a 'save' method on top-level document's that uses the collection
manager's '_save_document' method to save the document to the database.
Added tests for CollectionManagers -- all unit tests now require a valid
connection to the database, which is set up in the tests' setUp method.
Included tests for required fields (checks primary key is required
as well).
Switched from using SuperClass.__init__ to super().
Added constructor for EmbeddedDocumentField.
