Add initial patch for PUT and DELETE http methods

Only `put_service` and `delete_service` are implemented for now.
Add co-services for HTTP methods PUT and DELETE
Fix external_service_{put,delete} types
Update API documentation for PUT and DELETE services
Update PUT and DELETE services to be more specific [WIP]
Fix typing of PUT and DELETE services
Merge pull request #59 from venator/http_put_delete

Services for HTTP methods PUT and DELETE