Do a purge on files sent through syncstatic

The python cloudfiles API supports a purge command that clear the CDN entries and let apply changes faster:

https://github.com/rackspace/python-cloudfiles/commit/0c5f6e46bfc53230828e2a1554406ad6799ca1d6

I created an option to the syncstatic command to purge the CDN entries for the files sent through it.