Load dcmjs.js asynchronously and display progress bar

* Update dcmjs.js based on 1c35a46
* Add pace.js module with "loading bar" theme
* By default, hide "#main_content" section
* Show the "#main_content" section after all resources are loaded
Merge branch 'load-dcmjs-async' into gh-pages

* load-dcmjs-async:
  Load dcmjs.js asynchronously and display progress bar