v0.5.0

This release brings a couple of improvements to how cookbook dependencies are resolved, making it possible to use chef-runner as a "general purpose Chef provisioner" outside of a cookbook directory. (Thanks to @guilhem for the idea!)

For this, chef-runner now follows these rules:

• If the current directory contains a Berksfile, Berkshelf is used to manage
  cookbooks.
• If the current directory contains a Cheffile, Librarian-Chef is used to
  manage cookbooks.
• If the current directory is a cookbook with a metadata.rb file that defines
  the cookbook's name, only this cookbook is copied to the right place.
• The same is done when dependencies have already been resolved with Berkshelf
  or Librarian-Chef to reduce overall provisioning time.
• metadata.rb is only required when defining recipes in a format other than
  cookbook::recipe. The standard syntax cookbook::recipe should always work.
• chef-runner fails if no cookbooks can be found.

Other improvements:

• Configure Chef to verify all HTTPS connections. This removes the SSL warning
  message printed at the start of every Chef run. (Internally, ssl_verify_mode
  is set to :verify_peer, which is going to be the new default someday.)
• Add script/coverage to generate code coverage statistics for Go packages.
• Test dependency handling and provisioning more thoughtfully.
• Log most executed commands when log level is set to debug.
• Improve many parts of the README.
• Add Go documentation to some packages.