#2 Support for custom 'p4 resolve' handlling

[Lewis Baker]

Running the 'p4 resolve' command calls back to the
ClientUser.resolve() method with a ClientMerge object
that contains the details of the resolve operation.

However, the ClientMerge class just has a stub
implementation and so isn't very useful.

It would be more useful if the ClientMerge class was
fully implemented so that you could perform custom
operations or launch other programs from Python to
handle resolving file conflicts.