immutable AttrDict instances

[timodonnell]

I find I need an immutable mapping fairly often. Would it be reasonable to add an attrdict.FrozenAttrDict class that supports __hash__?

[bcj]

It's unlikely I will implement this (especially since I've pretty much ignored this project for the last 4 months), but I would definitely accept a pull request (based on the 2.0 branch).

I'm going to keep this issue open as I do wish Python had a frozendict, and might feel ambitious enough to give it a try in the future.