Support OS X .plist Files for Environment Configuration

[groboclown]

The existing environment configuration code reads from environment variables and Windows registry, but does not look at the OSX .plist files.

The following changes will need to be done to support this feature:

• PlistParser - Currently empty. Will need implementation to extract data for the Perforce settings.
• OsXPreferencesPart - Currently empty. Will need to implement ConfigPart, using the PlistParser to make the Perforce settings available.
• EnvCompositePart - The OsXPreferencesPart should be added in where the existing windows registry part is used.
• Documentation - The main page's REAMDE.md will need to denote support for this, and the TODO.md will need to be updated to remove this item.