Bug 800402 - [email] UI: Show the number of unread messages in the message-list header after the folder name

Please add a comment block that explains our semantics for unread here.  The goal is that someone reading the mailapi.js as our public interface won't have to go digging into the back-end to figure stuff out.  And eventually that we'll have jsdoc producing usable output from this.  When that happens we'll have to move the comment up, probably, but that's fairly mechanical.