Now executable! Just run `tools/tagmessages`.
Also, get the username and password from a `.transifexrc` file.
And hardcode the project slug to `zulip-test` rather than to `zulip`;
the Transifex API is bad at namespacing, so this makes it possible to
run this script on a test project (the only way we're currently using
it) even for people like me who can also upload to the real Zulip
project on Transifex.