Allow #fetch access in Rash

This allows developers to use the #fetch method on a Rash,  the idea being that it either raises a `KeyError` or `yields` if there is nothing that matched the given key.  Also defines `respond_to_missing?` on the Rash class, since it was missing that.

On a side note, I keep saying find instead of fetch.  I don't know why.  :cake:
This fixes a bug, right? Needs tests.