refactor(client): FxaClient no longer takes a relier in its constructor.

@ckarlof - This is the FxaClient without the relier passed in to the constructor. The core of this update was pretty easy, updating the tests was not. Since so many fxaClient.signUp calls has to be updated in the unit tests, I replaced most calls to the auth-server with sinon stubs.

* Instead of creating the FxaClient with a relier in its constructor, pass a relier instance to the functions that need it.
* Since we had to update any unit test that used fxaClient.signUp to pass in a relier, it was easy to update the tests to no longer touch the server.
* Increase the unit test coverage of fxa-client.js and complete_sign_up.js
* FxaClient.signUp no longer calls .signIn, the sign up view takes care of this.

Note, no functional tests were modified with this!

This is not a full fix for the issues listed in #1777, but contains probably the most difficult part.

issue #1777 


> Should sessionTokenContext come from the relier now?

Makes sense, unless it's being read from an existing session token (context).