Minimongoid should support modifiers on update

[sslotsky]

Meteor's collection.update allows a modifier argument (http://docs.meteor.com/#update)

This allows us to use MongoDB features like $addToSet (http://docs.mongodb.org/manual/reference/operator/addToSet/#op._S_addToSet)

Because Minimongoid's update method only takes a set of attributes and then calls save, the only way to achieve $addToSet is to copy the set, add to the copy, and then add the copy to the attributes.

Minimongoid methods would ideally allow for the same arguments that are allowed by Meteor.