Add Transifex client configuration.

This will enable users to pull in updated translations using the
Transifex client, which is useful for e.g. automating this process.

More about the Transifex client here:
http://support.transifex.com/customer/portal/topics/440187-transifex-client
Merge pull request #224 from Freso/transifex-config

Add Transifex client configuration.