Support ability to add certificate exceptions.

[nolanl]

Lots of folks have self-signed certs that they manually added exceptions for, and lots of these certs don't have a correct CN field. This was no problem in the pre-convergence world, but causes cert failures post.

[cless]

Relying on firefox 'native' exceptions while using convergence is not a good idea because convergence returns its own certificate. The more correct way to get around this is implement fingerprint exceptions in convergence itself. There is a fork at https://github.com/melknin/Convergence that has cache management. It has been discussed a bit in #7, you might want to read that discussion before trying it.

[moxie0]

It turns out that there are some security implications to setting the CN field to what FF would like it to be, so we'll have to implement a smooth exception interface built into the FF dialog that renders out instead.