Load javascript asynchronously

Previously the HTML output loaded the javascript synchronously.  This
caused the page to pause for many seconds if search_index.js was very
large (2.3MB for ruby 2.1).

Now jquery is used to load the search_index.js and other files
asynchronously so the page content displays quickly.
Merge pull request #306 from Elffers/master

Ignore generated doc directory
Add #306, #305 to History