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/document.py

@@ -5,22 +5,18 @@ import warnings
 
 from datetime import datetime
 
-import pymongo
-import pickle
-import weakref
-
 from fixtures import Base, Mixin, PickleEmbedded, PickleTest
 
 from mongoengine import *
-from mongoengine.base import _document_registry, NotRegistered, InvalidDocumentError
-from mongoengine.connection import _get_db
+from mongoengine.base import NotRegistered, InvalidDocumentError
+from mongoengine.connection import get_db
 
 
 class DocumentTest(unittest.TestCase):
 
     def setUp(self):
         connect(db='mongoenginetest')
-        self.db = _get_db()
+        self.db = get_db()
 
         class Person(Document):
             name = StringField()