Use a variable rather than constructor argument for allow_shutdown.

This is consistent with the other options, and makes it easy to see all
the settings.
Report config errors, rather than assertions in IRCClient.
Fix whitespace.
Remove no-op addFactoryArguments calls.

addFactoryArguments is deprecated now.
s/validate_master_option/validateMasterOption/
convert calls to addStep in factory.py to use instances
Merge remote-tracking branch 'origin/buildbot-0.8.8' into buildbot-0.8.8
relnotes for bug 2502
Merge remote-tracking branch 'org/buildbot-0.8.8' into buildbot-0.8.8