StatsViewController updated to not force use of default WP.com account

Still need to fix race condition when signing into WP.com for the Jetpack blog
Use the main context instead of background when creating an account with LoginViewController
Don't dismiss the jetpack login controller until the operation is complete
Removed unused method in WPAccount that doesn't specify a context
Merge pull request #1333 from wordpress-mobile/issue/1300-jetpack-multiple-wpcom

Allows Jetpack sites that aren't using default WP.com account