Added REST support for blog syncing

Note that the syncBlogsForAccount: method is only called on the default
WordPress.com account, so we might as well drop XML-RPC support for it.

Refs #920 #1521

*Warning:* Jetpack sites are returned by me/sites but they won't work.
There's a bug in the API that returns the xmlrpc URLs as HTTPS, and we
don't even have the self-hosted credentials. We'll need to filter those
out until we fully support REST.
Filter out Jetpack sites from me/sites

They wouldn't work anyway since we don't have the credentials for the
site
No need to wait for merge blogs block

Since we're already passing the completion block to mergeBlogs: there's
no need to perform the block synchronously
Merge branch 'develop' into issue/1521-rest-account
Merge pull request #1676 from wordpress-mobile/issue/1521-rest-account

Added REST support for blog syncing