rename base Thor task to Base
make --local and --no-local global thor task options

1. if --local: bundle exec [gem]
   special case: 'bundle exec bin/sitediff'
2. if --no-local: [gem]

Defaults to --local.
remove unused code
add a default spec task to run both unit and fixture tests
update Gemfile, circle.yml, and .travis.yml to use Thor