Bug 837269 - Remove "Sent from my Firefox OS" tag line from email app. r=squib

https://bugzilla.mozilla.org/show_bug.cgi?id=837269
land https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/123
Merge pull request #8069 from asutherland/email-remove-default-sig

Bug 837269 - Remove "Sent from my Firefox OS" tag line from email app. r=squib