Added multidb support

No change required to upgrade to multiple databases. Aliases are used
to describe the database and these can be manually registered or fall
through to a default alias using connect.

Made get_connection and get_db first class members of the connection class.
Old style _get_connection and _get_db still supported.

Refs: #84 #87 #93 #215

tests/connection.py

@@ -0,0 +1,48 @@
+import unittest
+import pymongo
+
+import mongoengine.connection
+
+from mongoengine import *
+from mongoengine.connection import get_db, get_connection
+
+
+class ConnectionTest(unittest.TestCase):
+
+    def tearDown(self):
+        mongoengine.connection._connection_settings = {}
+        mongoengine.connection._connections = {}
+        mongoengine.connection._dbs = {}
+
+    def test_connect(self):
+        """Ensure that the connect() method works properly.
+        """
+        connect('mongoenginetest')
+
+        conn = get_connection()
+        self.assertTrue(isinstance(conn, pymongo.connection.Connection))
+
+        db = get_db()
+        self.assertTrue(isinstance(db, pymongo.database.Database))
+        self.assertEqual(db.name, 'mongoenginetest')
+
+        connect('mongoenginetest2', alias='testdb')
+        conn = get_connection('testdb')
+        self.assertTrue(isinstance(conn, pymongo.connection.Connection))
+
+    def test_register_connection(self):
+        """Ensure that connections with different aliases may be registered.
+        """
+        register_connection('testdb', 'mongoenginetest2')
+
+        self.assertRaises(ConnectionError, get_connection)
+        conn = get_connection('testdb')
+        self.assertTrue(isinstance(conn, pymongo.connection.Connection))
+
+        db = get_db('testdb')
+        self.assertTrue(isinstance(db, pymongo.database.Database))
+        self.assertEqual(db.name, 'mongoenginetest2')
+
+
+if __name__ == '__main__':
+    unittest.main()