Add ability to pause build-slaves.

Signed-off-by: Tom Prince <tom.prince@ualberta.net>
Start implementing option to pause a slave.

Signed-off-by: Tom Prince <tom.prince@ualberta.net>
- New WIP for Clean shutdown.
Monkey patch recent gatherResults on old versions of twisted.

Newer versions of gatherResults support consumeErrors, which we would like to use.
Merge branch 'gatherResults'
- Changed graceful to clean.  Added also some docs.
Handle slave disconnections during build start better

This:
 - checks for a slave disconnection at the last possible moment
 - handles exceptions from startBuild's deferred better
 - treats errors in startBuild as EXCEPTION, not FAILURE

Fixes #2427.
comment about maybeDeferred use; better log.err message
Reference relevant bug.
Merge pull request #612 from djmitche/bug2427

Handle slave disconnections during build start better.
Use eventual.eventually more consistently.
- Fixed test_scripts_stop.py.  Changed short option from g to c. Fixed subfunction s/sighup/sigusr1
Merge branch 'eventually' of git://github.com/tomprince/buildbot
- Fixed stop_clean.
s/Shutdowns/Shuts down/
- Changed SIGHUP to SIGUSR1.
- Fixed documentation to reflect the new --clean parameter for stop.
Don't react to "destroy buildbot"
s/not/!=
Check against configurable nickname