Use REST to sync blog options

A small start but a start nonetheless.

This adds a basic to start implementing incremental REST support for `BlogServiceRemote`.

It also syncs blog options with REST when possible. Note that most of the times options are synced in batched requests and those will still use XML-RPC. 

Also, I've left out all the media size options since we're not using them anyway.

Also missing from REST is the Jetpack version that we use for `-[Blog hasJetpack]`. For now, all Jetpack sites will use XML-RPC, but it needs to be fixed before we switch them all to REST.

Refs #1521