return deferreds from addLog, addCompleteLog, and log.add*
return deferreds from addHTMLLog and addURL
cmd.add* return deferreds
backport ShellMixin and CommandMixin from nine
add step.setStateStrings
add docs for setStateStrings
add step.*Statistic methods for new-style steps
prohibit use of step.step_status in new-style steps
warn on use of old-style log methods in new-style steps
be sure run() returns a status and doesn't call finished or failed
pep8
Merge djmitche/buildbot:8/new-style-steps (PR #1085)

+autopep8, pyflakes