Refactor p4 invocation from PerforceClient, and add unit tests.

In an effort to clean up PerforceClient a little and make it testable,
this change consolidates all the p4 invocation into a class called
P4Wrapper. A P4Wrapper class is passed to PerforceClient (defaults to
P4Wrapper, but unit tests can override this), and an instance will be
created from this. Callers can call into this and just deal with the
results, regardless of how p4 is called.

This allowed some unit tests to be written. While this is by no means
comprehensive, it does add some basic testing for things like old-style
and new-style Perforce counters, get_repository_info, and basic diff
building.

I'll be making use of these new tests for Perforce moved file support.

Going forward, we probably should look into using p4 -G to get
marshalled Python objects as results from the commands (in order to
avoid doing our own parsing), and add some exceptions for failure so
that PerforceClient doesn't have to parse error strings.

Unit tests pass.

I played around with post-review a little with a Perforce client and
didn't see any regressions, but further, more real-world testing would
be helpful.