Add transifex client configuration for project.

See http://docs.transifex.com/developer/client/ for the documentation
on the client. The two basic actions we need are:

    tx push -s    Pushes a updated source translation to transifex
    tx pull -af   Downloads all current translation overwriting the
                  existing ones.

Completely new translations have to be added manually to the
mumble.pro file to be picked up during compilation.