Don't add Devise config twice.

This substition was being made twice by the `install` generator,
resulting in two config parameters. Now we only add the config
if there is none already.
Restore 3.2 compatibility.

`param_key` caused spec failures on Rails 3.2, stemming from trying
to make a constant out of "chocolate i l0ve you!"

Converting the Name to a string and doing our own substitutions
seems to bypass the problem.