Update dateformatter.js

Corrected date calculation for day of week
Added/modified day of week tests

This matches PHP:
echo date('N', strtotime('2011-09-06T16:05:02.000Z'))."\n";
echo date('N', strtotime('2011-09-04T16:05:02.000Z'))."\n";
Merge pull request #375 from cyberwombat/master

Fix date formatting for 'N' (ISO-8601 numeric representation of the day of the week)