Robocopy build step on Windows, timezone support for P4Source

Robocopy: a wrapper around the `ShellCommand` to run `robocopy` on Windows.

P4Source: added support for specifying a timezone for the P4 server so we can convert times reported in the changelists to local times. This will make changes coming from foreign P4 servers show up in the right place in the Waterfall, for example.