Raj/4 digits as year

Parse four-digits as Year (YYYY) not time (HHmm)
For eg: treats  '2014'  as Year 2014 rather than Military Time 20:14
 - changed DateParser.g to recognize  'formal_year_four_digits' (without the month component) as formal_date
 - changed DateWalker.g to create 'explicit_year_only_date' from the above and made an alternative for date rule