Add a broker

This if for @nchapman. This contains #1606 so contains both the first Relier and the first Broker. See what you think!
This seems awfully specific. Can we tweak this to use the `Constants.FX_DESKTOP_CONTEXT` const?

```sh
$ grep -irn "fx_desktop_v1" . | grep -v "./\(dist\|.tmp\)/"
./app/scripts/lib/constants.js:15:    FX_DESKTOP_CONTEXT: 'fx_desktop_v1',
./tests/server/metrics-collector-stderr.js:60:      assert.equal(loggedMetrics.context, 'fx_desktop_v1');
./tests/server/metrics-collector-stderr.js:89:      context: 'fx_desktop_v1',
./tests/tools/firefox_profile_creator.js:26:  myProfile.setPreference('identity.fxaccounts.remote.force_auth.uri', profile.fxaContentRoot + 'force_auth?service=sync&context=fx_desktop_v1');
./tests/tools/firefox_profile_creator.js:27:  myProfile.setPreference('identity.fxaccounts.remote.signin.uri', profile.fxaContentRoot + 'signin?service=sync&context=fx_desktop_v1');
./tests/tools/firefox_profile_creator.js:28:  myProfile.setPreference('identity.fxaccounts.remote.signup.uri', profile.fxaContentRoot + 'signup?service=sync&context=fx_desktop_v1');

$ grep -irn "Constants.FX_DESKTOP_CONTEXT" . | grep -v "./\(dist\|.tmp\)/"
./app/scripts/lib/session.js:140:      return this.get('context') === Constants.FX_DESKTOP_CONTEXT;
./app/scripts/views/settings.js:28:        showSignOut: Session.get('sessionTokenContext') !== Constants.FX_DESKTOP_CONTEXT
./app/scripts/views/sign_in.js:106:      if (Session.get('context') !== Constants.FX_DESKTOP_CONTEXT) {
./app/tests/spec/lib/app-start.js:29:      return '?context=' + Constants.FX_DESKTOP_CONTEXT;
./app/tests/spec/lib/router.js:67:        windowMock.location.search = '?context=' + Constants.FX_DESKTOP_CONTEXT;
./app/tests/spec/lib/router.js:69:        assert.equal(navigateUrl, '/forgot?context=' + Constants.FX_DESKTOP_CONTEXT);
./app/tests/spec/views/settings.js:78:          Session.set('sessionTokenContext', Constants.FX_DESKTOP_CONTEXT);
./tests/functional/settings.js:80:        .get(require.toUrl(SIGNIN_URL + '?context=' + Constants.FX_DESKTOP_CONTEXT))
```