Removed the dependency on the "diff" command, and replaced with support for Python's difflib module.
This requires a bit of a hack to work around an issue in difflib with handling files that don't end in newlines: http://bugs.python.org/issue2142 (this was fixed in Python 2.6, but the fix doesn't seem to be there in 2.4 or 2.5).

Also added a fix for a minor issue in PerforceClient._depot_to_local() where the local path sometimes has a trailing newline character.

Used by a few people successfully for a couple days on Windows and Linux systems.