Update validate group argument to options object

[ricardogama]

This PR updates the last group argument of Validator.validate method to be an options object. To validate with assertion groups, the group option can be provided on the options argument:

Validator.validate({ foo: 'bar' }, {
  foo: [is('bar').equalTo('bar'), is('biz').equalTo('biz')]
}, {
  group: 'bar'
});

This is a breaking change but will enable adding more optional features in a scalable manner, as the ones proposed on #60 and #61.