Default to None for values which don't exist in lookup_data

Basically an alternate solution to https://github.com/toastdriven/restless/pull/30

Erroring out of the loop of fields for lookup_data makes it hard to properly catch errors without overriding the entire prepare method (which is fine, but forces people to look at the source code (which is also fine haha)). The problem with this solution is that it does not fail so people will not know if they misspelled their fields, they will need to realize this based on the field they are looking for being ``null`` when it really shouldn't be.

My alternate solution (which I have yet to implement and is still brewing in my mind) is to be able to hook into the for loop in the prepare method so that errors to it could be handled on a per-field basis.