parse `ObjectID` strings

Fix for the recursive toJSON calling in base modella which was
converting `ObjectID`s into strings before the save method is called.

Setting `{type: ObjectID}` on an attribute will cause any string value
passed in to be parsed into an `ObjectID`.
updated to newest modella for testing
do proper checking
also supprt `'ObjectId'` and `'ObjectID'` as types for parsing
Merge pull request #20 from paulbdavis/parse-objectid

Parse objectid