Added support for setting a custom TTL on the container

I noticed that the TTL setting in django-cumulus wasn't used. Is there a reason for this? python-cloudfiles sets it anyway (to 86400), even if the user doesn't specify it.

I re-enabled the setting. If it is not set, the default is used, which is identical to the previous behaviour.

I'm using django-cumulus for static files (CSS, JS etc.), so I'd like a higher TTL set.

Also updated the docs accordingly.