Support for Mongo ObjectIDs

[krishaamer]

Orion currently only supports mongodb "_id" as strings, however by default MongoDB generates ObjectIDs (for example when using mongoimport). As Orion uses the _id as a slug to access single documents, this breaks the admin navigation.

https://devillex.com/using-mongoimport-with-meteor-and-orion-cms/
https://groups.google.com/forum/#!msg/meteor-talk/f-ljBdZOwPk/oQYZQxCAKN8J

[krishaamer]

@rwatts3 @nicolaslopezj Do you plan a doing fix or should I make a fork?