Add validation contexts

Validation contexts allow you to have multiple validation rules
that apply to different contexts within the same instance.

Example:

  var validator = app.validator();

  validator.context('foo', function() {
    this.add({name: 'alpha'}, "Invalid name");
    this.filter({
      name: this.fn.toUpperCase
    });
  });

  validator.context('foo').validate(fields);