Validate source options for cookbooks

Invalid source options in a Policyfile.rb will now be detected as the Policyfile.rb is read and reported as errors. To keep the tests passing I had to disable tests that specified chef-server as a cookbook source, since that isn't implemented in cookbook-omnifetch (and we probably won't implement that until Chef Server adds the /universe endpoint).
Maybe? I don't really get the utility of the way `pending` behaves in rspec 3, but if you think it's super awesome, maybe you can explain why?