Work in progress: Gaia buildtime bindings for l20n.js

[Staś Małolepszy]

I was able to plug l20n into the Gaia build system. As a proof of concept,
I started with the pretranslation:

https://github.com/l20n/l20n.js/compare/v3.x...stasm:gaiabuild
https://github.com/mozilla-b2g/gaia/compare/master...stasm:1169361-build-async

The next step is adding the getAST method to the build bindings. After
this is done, we'll be able to remove the need of loading the l10n script
into a mock window during build and simply require() it!

Depending on how our AST evolves, we might need to maintain both l10n.js
and l20n.js methods in webapp-optimize. OTOH,if we chose to port future
AST changes from v3 to v2, we might be able to only use l20n.js in build.

-stas