DISPATCH-52: Integrate system_tests_broker, fix outstanding issues.

system_tests_broker:
- Finish 3 node simple workqueue test, passing. Add to qdtest.sh.
- Skip tests if requirements (qpidd, qpid_messaging etc.) not available.
- Start brokers in setUpClass, re-use for all tests (currently only 1)

system_test:
- Add auto-flush to Messenger
- Support for setUpClass, tearDownClass including workaround for python 2.6

Developer doc:
- Add overview description of waypoints

Ran pylint on both

git-svn-id: https://svn.apache.org/repos/asf/qpid/dispatch/trunk@1594967 13f79535-47bb-0310-9956-ffa450edef68