Bug 1014219 - [Messages] Consider using real <button>s instead of <a> links for the header icons

The settings button on header, is changed from <a>  to button element  and button id has been changed according to the convention.
Total changes are done on 2 files.
Could you please make one more little change in this patch? I think it would be better to change id of composer link from ```icon-add``` to ```threads-composer-link``` (+ related js change inside thread_list_ui.js + change id here https://github.com/mozilla-b2g/gaia/blob/b8e9fb908ae2631d6c67abbeff0afbe5e783cba9/apps/sms/test/marionette/lib/messages.js#L46).