Partial support for thousands separators in DateTimeParsers

Added support for thousands separators, but only for negative
years. Reason: "1.10.100" is most probably a "day.month.year"
after Christ but "-1,10,100" is more probably the year "110100"
before Christ with decimal grouping (this example uses Indian
decimal grouping).

Short:
* Positive years are typically 4 digits.
  It's rare to use separators in 4 digit numbers.
* Negative years are often longer than 4 digits.
  It's common to use separators in "1000000 B.C." and such.

Change-Id: I04cc07674b7f675652a9ed94e17dbfe2acb710a7