Revert "Temporarily remove translations."

This reverts commit 89973efacacc958d8ae48d85daf814782920fb7e.
Add a transifex-client (tx) configuration file.

This uses this Transifex project:
https://www.transifex.com/organization/murrayc/dashboard/android-galaxyzoo
We can then push translation to transifex.
Add a README_transifex_client file for when I forget how to use it.
README_translators: Mention that we use Transifex.
Pull translations from Transifex.

No strings were changed, but transifex adds the context comments
into the translated strings.xml, so doing this now will make
it easier to see what has changed after the next tx pull.