Action-Schema JSON validation

charm.ActionSpec now has a ValidateParams method which accepts an
unmarshaled JSON document, and validates it.  If it is valid, the
method returns true; otherwise, it returns false and a list of
errors returned by gojsonschema.

WIP, opened PR for comments
Unfortunately, Errors() returns a custom type for gojsonschema, ValidationError.  

http://godoc.org/github.com/binary132/gojsonschema#ValidationResult.Errors

ValidationError itself has a String() method.