Add support for fetching from Transifex

So, here's the first cut of being able to work with Transifex to manage translations.

I've added in [my library](https://github.com/BabDev/BabDev-Library) which has enough of the Transifex API in place to use it here and a new CLI command, `get transifex`, which will fetch translations.  Currently only the JTracker Core (`/src/JTracker/g11n`) is supported as that's all I've uploaded to my test project at https://www.transifex.com/projects/p/babdev-library-api-test-project/.

Added also are `transifex` options to the configuration JSON to authenticate and define the base project.

Still a lot to build out (like the rest of the get command, a push command, a project on Transifex, etc.), but it's enough of a teaser to get going :-)