Extract codename logic to a library

Cleans up #6 a bit.
Prefer single quotes
Use Chef::Log instead of log resource
Update chefspec dependency

v1.0.0 has been released. Lock the requirement to '~> 1.0'.
Also simplify chef requirement logic.
Add tailor tests
Add guard-foodcritic setup
Reorganize Rakefile

Copy ideas and code from https://github.com/mlafeldt/skeleton-cookbook
Add tests for backports recipe
Update changelog
Merge branch 'test-improvements'