Fully Support MongoDB Storage

[acouch]

The FileStorage subclass of the Content class uses the same storage methods as MongoDB. This concept is used in the almost identical Storage class in the admin https://github.com/interra/catalog-admin/blob/master/server/models/content.js#L112 . The non-mongo methods from FileStorage should be moved to the parent class and FileStorage should be implementing storage methods. Would also like to add tests for Mongo as well.