Add Authorizers to restrict access to objects

[alexbarbato]

We need to be able to assign functions to restrict access to certain object, potentially based on the Context.

Imagine for example I'm interacting with an auction website API. I may request the status of a particular auction, and as part of that object there is a list of 'bids' on the auction. I should only see my own, not bids belonging to anyone else.

I should be able to attach a function to the 'Bid' object type to restrict it from being projected (either as a property or within a collection) if my criteria isn't met.

[undiwahn]

I'm currently implementing this!

[alexbarbato]

Code completed and uploaded to master