Alias 'presence' validator as 'required'

[vdh]

Now personally, I don't mind that the original validator is named presence, but my co-workers are a tad antsy about their validation rules not matching what they're used to in other frameworks.

So I thought it would be handy to have an alias named required, for those who are more accustomed to writing validation that way, or prefer something a little harder to mis-type ("presense"/"precense").

[coveralls]

Coverage remained the same at 100.0% when pulling 7f712b8 on vdh:alias-presence-validator into 49891ac on ansman:master.

[loyd]

I use custom required validator to describe != null properties:

  validate({items: []}, {
    items: {
      required: true,
      array: {number: true}  // custom forEach checker
    }
  });