Add SetProperty build step
Rename SetProperty step in buildbot.steps.shell to SetPropertyFromCommand
maildir bug fixed, allowed maildir to be started and stopped repeatly

Fixes #2475
Merge branch 'maildir_bug' of git://github.com/jbotto/buildbot
changed the unit test to AssertEqual
Merge branch 'maildir_bug' of git://github.com/jbotto/buildbot
Merge branch 'pycon2013-sprints' of git://github.com/dividehex/buildbot

(relnotes edited)
Ticket #2226 - Added build step LogRenderable
Merge branch 'bug_2226' of git://github.com/jbotto/buildbot