Stop using BuildStepStatus when running steps
Update status string handling for most tests

There are still some failing tests due to a lingering issue around
BuildStep no longer supporting a `describe` method.
add more compatibility for old-style steps

* BuildStep implements describe()
* Factor out command_to_string
Fix ShellSequence status strings; do error reporting in twistd.log
fix remaining tests
Remove the BuildSetStatus class, still allowing unpickling
remove LogFile and HTMLLogFile to buildbot.util.pickle
add tests for get{Current,Result}Summary