Basic Perforce provider

Should look and feel like the other SVN/HG/etc... providers.  I had to add a few Perforce specific parameters and support for Workspaces.  I may need to enhance the use of Workspaces later, but this should get most Perforce users started.
We recently updated the main README.markdown and removed all of the per-provider readmes. Could you please update the PR to incorporate this perforce readme into the various sections of the main readme?