Added mongoose document instantiation support...

...with population, lean, and population + lean support.

As mongoose-memcached works now the objects returned from a cached query are simple data objects with no mongoose document functionality, such as `save`, `delete`, `update`, and so on. This patch changes that and gives back, by default, mongoose document instantiated cached data, with support for lean, populate and a combination of both.
Note: This is written to support `findOne` query as well.