set .master for buildsteps
Introduce a 'run' method for steps, which simply returns the result

This does away with the complex need to call self.finished or
self.failed
Integration tests for custom buildsteps

This includes updates to fakes and interface tests for them
support catching deferreds from several BuildStep methods when old-style steps call them synchronously

Specifically:
 * addCompleteLog
 * addHTMLLog
 * addURL
add a process Log class hierarchy to interface with steps
(WIP) document rewriting steps in relnotes, rewrite steps
temporary TODO.txt
(WIP) new addLog API - but still talking to status

..so don't call data API

Also:
 - addCompleteLog
 - addHTMLLog
addLog: implement smooth transition for custom buildstep

+ Fix remaining tests

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>