WIP: Hook up the new FxA JS client.

* Update bower config to fetch the new FxA JS client.
* In postinstall npm command, install the FxA JS client's dependencies and do a build so the client is available.
* Create a light wrapper around the FxaJSClient to reduce boilerplate code.

@zaach - I could use some help figuring out how to update the `tests/functional/signIn.js` test, I have been so far unable to figure out how to include the new client into the tests.