Use mixin class for UnitTestMerge and UnitTestSetCommitStatus

This commit allows to group tests into without running into errors while
unittest tries to instantiate an abstract class (see also
http://bugs.python.org/issue17519)
merge master -D org -I include -E exclude: PR 82 (Add logic to create tag while updating submodules)
merge master -D org -I include -E exclude: PR 87 (Refactor the TestRebase integration test)