Add custom attribute label validator

[sewil]

Adds support for using attribute labels to be displayed in the error message instead of prettified attribute. This enables error message localisation. Usage:

const validationConstraints = {
  age: {
    attributeLabel: "The user age",
    presence: true,
    type: "string"
  }
};

Output: The user age is required

Changes:

• attributeLabel validator is bypassed in validator check loop.
• If errorInfo attributeLabel is set in convertErrorMessages, it will be prepended in place of prettified errorInfo attribute.

[coveralls]

Coverage remained the same at 100.0% when pulling bf246b6 on app-shack:master into c64df5d on ansman:master.