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.

tests/fields.py

@@ -5,6 +5,9 @@ from mongomap import *
 
 class FieldTest(unittest.TestCase):
 
+    def setUp(self):
+        connect(db='mongomaptest')
+
     def test_default_values(self):
         """Ensure that default field values are used when creating a document.
         """