Parse objectid

[paulbdavis]

Fix for the recursive toJSON calling in base modella which was
converting ObjectIDs 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.

[coveralls]

Coverage remained the same when pulling 38affa4 on paulbdavis:parse-objectid into bfd1b77 on modella:master.

[paulbdavis]

Rebased with conflict fix, should be good to merge this now

[coveralls]

Coverage remained the same when pulling 573ef86 on paulbdavis:parse-objectid into bfd1b77 on modella:master.

[rschmukler]

This looks good but would also like to see string support, ie 'ObjectId' should also work.

[paulbdavis]

Noted, will push that in a second

[coveralls]

Coverage remained the same when pulling 4e7b827 on paulbdavis:parse-objectid into 70f7d20 on modella:master.

[coveralls]

Coverage remained the same when pulling 1a4ed58 on paulbdavis:parse-objectid into 70f7d20 on modella:master.