Adjust other references to the old 'gitblit' organisation on Github to
point to the new 'gitblit-org' organisation.

The link to the nightly docker images points to the actual image layer,
which is removed when a new nightly is pushed.
Instead, we should point to the images page, filtering on the tag 'nightly'.

And, link to the contributing guidelines from the Readme.

Since Google Plus doesn't exist any more...

Change-Id: I717e13d1f852a4adf50b547f4769dd62da2384ce

* Build script overhaul including building & publishing GO, WAR, Docs,
and Site.
* Restored JGit 0.12.1 dependency and backported Blame.  Got tired of
waiting for JGit 1.0.0 Maven artifacts.
* Changed Summary Page layout
* Optional cookie authentication
* Added icons for log, tags, and branches panels.
* Show last commit author and short message on branches panel.
* Unit testing.
* Documentation.