add tx-pull and tx-push FILE helper scripts

tx-pull fetches translations from Transifex to .tx/<ressource>/<lang>
and converts the JSON into an AMD module. Requires `transifex-client`
from PyPi.

tx-push FILE submits a AMD translation module to Transifex using `curl`
and credentials from ~/.transifexrc.
add Transifex project configuration file
update translations from Transifex (initial pull)
add Transifex translation to contributing guide
Merge branch 'transifex'