Use HTML5 async attribute to speed up page loading.

Turbolinks requires we put the <script> tag in the <head>, but this
causes page rendering to block until the script has been downloaded
and executed. We can mitigate this by adding the async attribute to
the <script> tag.

See: http://www.html5rocks.com/en/tutorials/speed/script-loading/
Merge branch 'master' into bootstrap