Fix typos in documentation
Document class so I know what it is
Set removal date for DependencyInstaller#gather_dependencies
Add VendorSet and VendorSpecification

This is groundwork for supporting the path option in gem dependency
files.  Further modifications and enhancements may be needed as
support progresses.
Test Gem::RequestSet#load_gemdeps
Support the :path option to GemDependencyAPI#gem

Now the specification from the given directory is loaded into a
VendorSet.  It isn't used by the resolver yet, though.