Extend gverify comparison

- Allow comparing to a sepcific 'golden' manifest with `--compare-to`
- By default pick the first manifest to compare to, instead of always
  comparing against the previous one, which is confusing
- Show line-by-line difference if `-v` given
Merge pull request #64 from theuni/cache

cache fixes
Merge pull request #58 from laanwj/master

Extend gverify comparison
fix: use don't use methods that require new ruby version

Dir.exists isn't available in ruby 1.8