Fixes #21, add build step for mozilla-central

[dmose]

This adds a build step and creates an artifact for use as a JSM in mozilla-central.

This uses pretty much exactly the code from Normandy (including the same ancient versions of the dependencies). That's clearly worth fixing, and probably wants a spin-off ticket from this one, but the point of this ticket is really just to be able to build something from source that's similar enough to the current thing. Migrating a couple of major versions of webpack can happen in another ticket.

This ticket also stops pre-minifying the artifact, as it's going to be compressed in the omnijar in Firefox anyway, and removes unneeded dependencies.

To test:

$ yarn install 
$ yarn test
$ yarn build
$ cp vendor/mozjexl.js ../mozilla-central/toolkit/components/utils/
$ cd !$
$ ./mach build
$ ./mach test toolkit/components/messaging-system/targeting/test/unit

[dmose]

Still need to add a bit of documentation and steps for testing.

[dmose]

@brennie, I'd love a review so that it's possible to build stuff for m-c again. There are lots of things that could be fixed here; I feel like this is the right first step.

[freshstrangemusic]

r+ with some minor nits