Add 3rd party library highlight.js

Add 3rd party javascript library highlight.js
(http://softwaremaniacs.org/soft/highlight/en/). It is used for
highlighting syntax in code blocks.
Include script and styles in resource file

Include used script and styles from library highlight.js in resource file
resources.qrc.
Change links to highlight.js

Instead of linking to the online version of highlight.js, we use the
script and styles in the resource file resources.qrc.

This way the code highlighting works without internet connection.

Resolves issue #55.
Merge branch 'feature/offline-highlighting-#55' into develop