#1825 REST comments

Use REST when possible to sync, moderate, edit, or reply to comments.

I've been testing this with wp.com, jetpack and non-jetpack self-hosted, and everything seems to work fine, but feel free to look for corner cases and try to break it.

Fixes #1825 
Do you think perhaps this might be better off living in `RemoteComment` as an alternate initializer? i.e. `[RemoteComment alloc] initWithComment:comment]`?