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.
* Use the fxa-js-client from the bower install, get rid of picl-gherkin
Updates with @zaach's suggestions.

* Rename gherkin to fxaClient
* Use the special xmlhttprequest that supports .onerror
* Rename the FxaClient wrapper to FxaClientWrapper internally