Adding @FieldsEqual validation annotation

[creativedrewy]

I thought this annotation would be useful for forms where one or more fields need to be equal (i.e. password/confirm password).

There is a "fields" property on the annotation where you provide an array of the other field ids that must be equal to the annotated field for validation to pass.

Let me know if you want unit tests included before accepting the PR.

[creativedrewy]

@tomas-vondracek I've made the updates you described above. Let me know if there is anything else!

[tomas-vondracek]

Thank you!