add test for #458
add `clone` to dependencies in package.json
Gruntfile: add boilerplate for clone like there is for parserlib
clone ruleset before passing it to applyEmbeddedRuleset; fixes #458

since applyEmbeddedRuleset modifies the ruleset in-place,
this ensures that those modifications aren't visible to the caller
fix typos in comments in Gruntfile
Merge pull request #465 from cvrebert/defensive-copy

copy the passed-in ruleset in verify(); fixes #458