Rule methods should implement interface

[ankithans]

To make it easy for adding more custom rules to validator, rules methods should implement an interface.
we should have the same interface and the same function. Without that there will be no way to add rule to validator. This two methods should be the same method

Currently

lengthRule implements - validateLength
MustExistRule implements - validateMustExist

[wtrocki]

and they should be just validate