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

https://bugzilla.mozilla.org/show_bug.cgi?id=837269
r? @mozsquib

This patch removes the default entirely and updates the compose unit tests to assume there is no signature present by commenting out those bits.  We will obviously want some test coverage of signatures in the future, which is why I commented those bits out rather than nuking entirely.

This does not rev the database or otherwise attempt to clean up existing signatures; I'll ask on the bug about that.