Add l10n_completion.py script

This script will accumulate data in a JSON file for percent translated broken down by locale and then by app. If you run it twice, it'll "append" to the existing JSON file.

It's a first pass. I'm curious to see if this:

1. addresses needs
2. the output is in a format that's helpful

Now that I'm looking at it, I'm noticing it has no comments in it. That's lame. I'll fix that later.

Anyhow, feedback?