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/dynamic_document.py

@@ -1,13 +1,14 @@
 import unittest
 
 from mongoengine import *
-from mongoengine.connection import _get_db
+from mongoengine.connection import get_db
+
 
 class DynamicDocTest(unittest.TestCase):
 
     def setUp(self):
         connect(db='mongoenginetest')
-        self.db = _get_db()
+        self.db = get_db()
 
         class Person(DynamicDocument):
             name = StringField()