added validation to check orphans dob within gestation period of fathers death
 amended checks for gestation period to be within 1 year of fathers death

tidied up code, remove constants and made conditional more readable
changed validation method name for improved clarity
changed cucumber features to fit within new validation criteria
Merge pull request #34 from PurityControl/orphan_dob_validation

@sampritipanda: If you still think the date validations could be handled better, let's revisit the issue in the future.