Updated README.md

I've patched the application to avconv for Ubuntu using quilt so ffmpeg is no longer needed and the webupd8 team has generously let me copy their yad package into my PPA, so theirs isn't needed anymore.  `apt-get update` should be run after removing a PPA so our sources are up to date.
Merge branch 'Seth-Johnson-patch-1'
Merging Seth-Johnson's changes to README.md about easier Ubuntu install
Add support for Pantheon Desktop used in Elementary OS
Merge branch 'Arkni-pantheon-support'
including Arkni's support for the Pantheon desktop