add support for LibCloudStorage with Django1.7 migration features

I've tested this with Rackspace via ApacheLibCloud using the following settings:

DEFAULT_LIBCLOUD_PROVIDER = 'rs_test'
LIBCLOUD_PROVIDERS = {
    'rs_test': {
        'type': 'libcloud.storage.types.Provider.CLOUDFILES_US',
        'user': os.environ['RACKSPACE_USERNAME'],
        'key': os.environ['RACKSPACE_API_KEY'],
        'bucket': 'storages_test'
    },
}

I also tested that 'python manage.py makemigrations <appname>' works after making this change (it didn't before).