Update CONTRIBUTING.md to discuss Chef Zero
Move gems to gemspec
Start Chef Zero at suite, not before each run
Add shared knife.rb and testing pems
Switch to new Chef Zero API
Use stock knife.rb in tests
Revert "Move gems to gemspec"

This reverts commit d21e89573fe59dfbfb9193cba9bf7dfdfe73f93a.
Bump CZ dependency
Merge pull request #546 from sethvargo/chef_zero_and_specs

Fix CZ on master