use rfc1123 date format for weak_date

while headers are always sent using this format, it should be also used
when parsing the date
Merge branch 'fix-weak-date' of github.com:superdesk/eve into superdesk-fix-weak-date
Move RFC1123_DATE_FORMAT to __init__.py

So we can eventually use it elsewhere as needed.
weak_date always returns a RFC-1123 date.
Merge branch 'superdesk-fix-weak-date' into develop