Refactor the surrogate keys we've used
Refactor AttributeDict

* Moves AttributeDict into warehouse.datastructures
* Refactors convert_to_attr_dict to just be the __init__ for
  AttributeDict
Declare constants for the Fastly surrogate keys
Merge pull request #265 from dstufft/refactor-fastly-cache

Use a Standard Set of Fastly Surrogate Keys