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

Corresponding PR in gelam is here: https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/315
With the getElementsByClassName as an instance var, this function can turn into a prototype method, and we can avoid extra overhead of a function scope created on every call to adjustHeaderMargins.