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/collection.py

@@ -0,0 +1,18 @@
+from connection import _get_db
+
+class CollectionManager(object):
+    
+    def __init__(self, document):
+        """Set up the collection manager for a specific document.
+        """
+        db = _get_db()
+        self._document = document
+        self._collection_name = document._meta['collection']
+        # This will create the collection if it doesn't exist
+        self._collection = db[self._collection_name]
+        self._id_field = document._meta['object_id_field']
+
+    def _save_document(self, document):
+        """Save the provided document to the collection.
+        """
+        _id = self._collection.save(document)