Change DATE_FORMAT to include milliseconds

Created by: fubu

Currently the DATE_FORMAT is set to %Y-%m-%dT%H:%M:%SZ, which prevents JS clients to natively use their Date.toISOString() method to convert JS datetimes into strings for sending them with JSON. This is because JS always includes milliseconds in the ISO representation, even if they are set to 0:

d = new Date();
d.setMilliseconds(0);
d.toISOString();  # still includes milliseconds!

To simplify API clients (mainly JS-based ones at the moment) I propose to change the DATE_FORMAT to %Y-%m-%dT%H:%M:%S.%fZ

(The optimal solution would be to allow both formats, but unfortunately Eve does a simple datetime.datetime.strptime parsing with the configured DATE_FORMAT on incoming datetime strings, instead of using a real parser - which was proposed, but rejected.)