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>
Merge remote-tracking branch 'origin' into poc/pause

Conflicts:
	master/buildbot/status/web/authz.py
	master/buildbot/status/web/slaves.py
	master/docs/cfg-statustargets.texinfo
Make sure to start builds, when unpausing a slave.
Some fixes for updated auth system.
Add missing css class.
Improve display of slaves on builder page.
Merge pull request #623 from tomprince/poc/pause

RFC: Support for pausing a build slave.

Fixes #537.