mv github_status.rb github_status_publisher.rb
Fixup after rename.
Add help snippet for plugin.
Fix typo.
Remove custom table style.
Add test_credentials function to GithubApi to validate the instance's attributes.

This could be used for form validation in the future.
Remove the "Test Connection" validate button.
Alphabetize private methods.
Before running perform, run validation.

Closes issue #2