Integrated linkchecker into Travis Test

Integrated linkchecker into travis tests, and fixed broken links.
- hosting the website on Travis and run linkchecker to check errors there
  - set 10s timeout and any link that returns a timeout is treated as **valid** (to prevent some problems, e.g. freebase.com do not respond sometimes)
- fixed all broken links in this branch
- added ubuntu.html to version control (as ubuntu.md)