Use a single call to fetch stats instead of 13 separate ones

Closes #1686 

Switched to the /batch API on WordPress.com to fetch web stats.  Created new classes called WPStatsService and WPStatsServiceRemote to abstract out the fetching process.  Added some basic unit tests but could use a TON more, especially with the data mapping in the remote.

Still need to test Jetpack sites more with this but the logic is pretty close to the previous way it was done if it's a Jetpack site.