add unequalTo numericality constrainst

[codynguyen]

Hello! This adds support for inequality check when validating numericality.

Example:

const constrainsts = {
  value: {
    numericality: {
      unequalTo: 10
    }
  }
};

validate({value: 10}, constrainsts); // => {value: ["Value must be unequal to 10"]}

[coveralls]

Coverage remained the same at 100.0% when pulling 44444b1 on codynguyen:unequalTo into c64df5d on ansman:master.