Fixes #1402 by adding the jetpack_user_email param to the xmlrpc wp.getOptions endpoint and storing locally.

Testing

• Click on the Sign in button in the example app.
• Enter username, password and xmlrpc url for the site with Jetpack installed/activated/connected.
• Once successfully logged in, click on Sites -> Fetch first site.
• Verify that the xmlrpc wp.getOptions endpoint is called and the jetpack_user_email corresponding to the login credentials is received correctly.
• Login with a different jetpack connected user for the same site and verify that the jetpack_user_email is changed accordingly.

Note that the jetpack_user_email field would only be available in the API response if jetpack is installed, activated and connected.