update serif (change controls, fix links, change url)

The Serif theme was a little flaky previously
Merge pull request #487 from OwenVersteeg/master

update serif (controls, attribution URL, and fix <a> elements)
correct links in sky theme

They looked a little wonky if you used a G or a Y.
Fix links so they aren't cut off (in SCSS)
Fix links so they aren't cut off (in SCSS)
Merge branch 'master' of https://github.com/OwenVersteeg/reveal.js
recompile themes #488
readme typo (closes #495)
Merge remote-tracking branch 'upstream/master'