Add support for Thunderbird

The extension works fine on Thunderbird 24.5 (tested on Debian with Icedove 24.5.0-2), add it to the `install.rdf` file.  The `minVersion` has been chosen based on the presence of `nsILoginManager`.

I've not yet added new passwords to the keyring, as the passwords have been stored in the GNOME keyring by the `xul-ext-gnome-keyring` extension by @infinity0 ([infinity0/mozilla-gnome-keyring](https://github.com/infinity0/mozilla-gnome-keyring)).

Secondly, I've slightly cleaned up the `Makefile` to avoid a build failure if the `bin/` directory already exists and to cleanup only those files in `bin/` created by the `Makefile`.

Thanks for working on this alternative implementation!