Add all recent contributors to the AUTHORS list.
Bring over a better UnitTest class from Review Board.

This class lets us have docstrings that wrap lines for the unit test
summaries, instead of constraining them to a single line.

This does somewhat duplicate the purpose of RBTestBase in
rbtools/utils/, but that should maybe be split up into a set of mixins
or a new class, as that's really more for client tests and not that
useful for API tests.

Reviewed at https://reviews.reviewboard.org/r/5962/
Fix compatibility issues with older versions of Review Board.

Versions prior to 1.7.8 didn't have a 'url' field, causing
the 'absolute_url' property to fail at generated a suitable URL. We now
have a custom 'url' property on ReviewRequestResource that provides a
suitable default, just like 'absolute_url' does.

Likewise, versions of Review Board prior to 1.7.19 didn't have a
'mirror_path' field on the Repository resource, which broke a lookup. We
now conditionally get the 'mirror_path' instead of assuming it's there.
This is done instead of providing a custom property, in order to prevent
the resource from being misleading (since we can't generate any sane
default there).

Fixes bug #3348
Reviewed at https://reviews.reviewboard.org/r/5963/
Release RBTools 0.6.1.
Merge branch 'release-0.6.x'