Add support for ISO8601 Basic as well as Extended format datetime strings

### add support for ISO8601 Extended and Basic formatted datetime strings
for django-storages with Mezzanine deployed on Heroku and assets served from S3

s3boto.py backend claimed: 

    # Dates returned from S3's API look something like this:
    "Sun, 11 Mar 2012 17:01:41 GMT"

when in fact I was getting something like this:

    "2014-02-22T06:34:26.000Z"

for last_modified datetime strings when uploading images to Mezzanine's Media Library.