Added Windows-specific buildsteps module.
Right now it only contains a `Robocopy` step.
Ability to specify a timezone for a P4 server

The Perforce server used with `P4Source` may be in a different time zone
than the Buildbot master, which would cause, for example, changes to be
displayed in the wrong place on the Waterfall display.

This change lets you specify a timezone for the server. Changelist times
will be converted from that timezone to the local timezone before being
added to the Buildbot master.
Fixed Robocopy step status

Robocopy actually returns a combination of flags to indicate
success/warning/failure status.
Added information about Robocopy exit codes.
Added documentation about Robocopy step.
Removed docstring for Robocopy step.

All usage information is only in the documentation page now.
Documentation about timezone support in P4 changesource
Release notes for Robocopy and P4 changes.
Fixed P4 source unit tests on Windows

This is because P4 normalizes slashes/backslashes, so we need to set our
expectations right.
Added unit tests for Robocopy step.

Fixed whitespace.
Don't pass `/UNICODE` to Robocopy by default.
Added more information about Robocopy in the docs.
Fixed 2.5 incompatibility in p4 tests
Fixed p4 poller tests
fix bugs in handling unkown return codes, pyflakes

..and return Deferreds from test methods.
Merge branch 'pull728'