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

mongoengine/queryset.py

@@ -1,4 +1,4 @@
-from connection import _get_db
+from connection import get_db
 from mongoengine import signals
 
 import pprint
@@ -481,7 +481,7 @@ class QuerySet(object):
         if self._document not in QuerySet.__already_indexed:
 
             # Ensure collection exists
-            db = _get_db()
+            db = get_db()
             if self._collection_obj.name not in db.collection_names():
                 self._document._collection = None
                 self._collection_obj = self._document._get_collection()
@@ -1436,7 +1436,7 @@ class QuerySet(object):
         scope['query'] = query
         code = pymongo.code.Code(code, scope=scope)
 
-        db = _get_db()
+        db = get_db()
         return db.eval(code, *fields)
 
     def where(self, where_clause):