Adds comma between day-of-month and year for en-US locale.

Specifically:
- LL   => 'MMMM D, YYYY'
- LLL  => 'MMMM D, YYYY LT'
- LLLL => ''dddd, MMMM D, YYYY LT'

... and similarly for the lowercase variants.

The use of a comma between the day-of-month and year is standard
practice in US English. See:
- ICU docs [http://demo.icu-project.org/icu-bin/locexp?_=en_US]
  (under Date & Time Patterns)
- Chicago Manual of Style (by way of the Library of Congress)
  [http://lcweb2.loc.gov/ammem/ndlpedit/handbook/numberdate.html]
- Wikipedia [http://en.wikipedia.org/wiki/Date_and_time_notation_in_the_United_States]
Merge pull request #1784 from 97jaz/develop