update doc and add new JJB unit tests

Add documentation on how to generate documentation and run JJB unit tests.

Also added a unit tests for each category of plugins as examples for others
to follow when adding new JJB plugins.  I'm hoping this will help contributors
add tests for new plugins going forward.

Change-Id: I2ab90cdb37d0f3b1e947345d4291d2927b04a0c6

Conflicts:
	tests/general/test_general.py
	tests/publishers/test_publishers.py
	tests/scm/test_scm.py