Support for Bulk Delete

* Connection.BulkDelete()

    deletes named objects at once using bulk delete API.

* BulkDeleteResult

    stores results of BulkDelete operation.

The referenced specification is Rackspace Cloud Files API:
http://docs.rackspace.com/files/api/v1/cf-devguide/content/Bulk_Delete-d1e2338.html

Tested on Rackspace.

See #8 for discussions.