Allow #find access in Rash

This allows developers to use the #find 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 fetch instead of find.  I don't know why.  :cake:
Is this expected to yield on key and not on value?