Makefile: cleanup build rules

 - don't fail, if $(bin_dir) already exists
 - remove XPI file first, $(bin_dir) afterwards

Signed-off-by: Philipp A. Hartmann <pah@qo.cx>
install.rdf: add Thunderbird to supported applications

The extension works well on Thunderbird, too.

Signed-off-by: Philipp A. Hartmann <pah@qo.cx>
Merge pull request #3 from pah/topic/thunderbird

Add support for Thunderbird