Scaladoc: Load scripts at the bottom, and with a defer attribute

To improve latency on modern browsers (which supports defer) and old browsers:

 * https://www.webkit.org/blog/1395/running-scripts-in-webkit/
 * http://developer.yahoo.com/blogs/ydn/posts/2007/07/high_performanc_5/
Merge pull request #2321 from kzys/js-defer

Scaladoc: Load scripts at the bottom, and with a defer attribute