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 pull request #13 from superdesk/fix-weak-date

use rfc1123 date format for weak_date
If-Modified-Since disabled on collection endpoints.

Please see the discussion at #334
(https://github.com/nicolaiarocci/eve/issues/334) for the reasoning
behind this decision. Closes #334.
Move RFC1123_DATE_FORMAT to __init__.py

So we can eventually use it elsewhere as needed.
weak_date always returns a RFC-1123 date.