Added CollectionManager, made connection module

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.

mongomap/fields.py

@@ -61,7 +61,13 @@ class EmbeddedDocumentField(BaseField):
     def _to_python(self, value):
         return value
 
+    def _to_mongo(self, value):
+        return self.document._to_mongo(value)
+
     def _validate(self, value):
+        """Make sure that the document instance is an instance of the 
+        EmbeddedDocument subclass provided when the document was defined.
+        """
         if not isinstance(value, self.document):
             raise ValidationError('Invalid embedded document instance '
                                   'provided to an EmbeddedDocumentField')