Improve update site and description site to make the both more convenient to work with

[daniilyar]

1. Change link to update-site at http://sevntu-checkstyle.github.io/checkstyle-ast-eclipse-viewer to be plain-text instead of using hyperlink. Provide "Copy to clipboard" button near that link instead.
2. Add update-site link and "Install to Eclipse" button to both readme.md and description site.
3. Change fonts and colours of description site (http://sevntu-checkstyle.github.io/checkstyle-ast-eclipse-viewer) in order to make it more readable.