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 #58 from laanwj/master

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

This is a regression from ea24af10. Dir.exists isn't available in ruby 1.8
Merge pull request #68 from theuni/cache

fix: use don't use methods that require new ruby version
Fix issue #63: use lxc-execute instead of lxc-start
Merge pull request #69 from gurnec/lxc-execute

Fix issue #63: use lxc-execute instead of lxc-start
Add warning in release notes about lxc-execute
Default back to lxc-start, with option for lxc-execute
cache: require the enable_cache key to be set to true in order to use cache functionality
update docs for enable_cache option
Merge pull request #70 from theuni/cache

Enable cache option