JENKINS-16790: Add link from coverage column to coverage report for last completed build
Set color of link to not have unreadable text with darker colors, set
line-color of NA to black to make it visible, add more unit tests.
Use lastSuccessfulBuild to be in sync with what the percentage and color shows
Reuse code in more places, defer variable instantiation until really needed
Merge pull request #23 from centic9/JENKINS-16790

JENKINS-16790: Link from Dashboard to coverage report