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/collection.py

@@ -0,0 +1,33 @@
+import unittest
+import pymongo
+
+from mongomap.collection import CollectionManager
+from mongomap import *
+
+
+class CollectionManagerTest(unittest.TestCase):
+    
+    def setUp(self):
+        connect(db='mongotest')
+
+        class Person(Document):
+            name = StringField()
+            age = IntField()
+        self.Person = Person
+
+    def test_initialisation(self):
+        """Ensure that CollectionManager is correctly initialised.
+        """
+        class Person(Document):
+            name = StringField()
+            age = IntField()
+
+        self.assertTrue(isinstance(Person.collection, CollectionManager))
+        self.assertEqual(Person.collection._collection_name, 
+                         Person._meta['collection'])
+        self.assertTrue(isinstance(Person.collection._collection,
+                                   pymongo.collection.Collection))
+
+
+if __name__ == '__main__':
+    unittest.main()