sha1 paste

Fixes #77.

Needs:

```sql
DELIMITER @@

DROP PROCEDURE IF EXISTS paste_get_content_digest@@
CREATE PROCEDURE paste_get_content_digest (
  p_digest BINARY(20),
  OUT p_content MEDIUMBLOB
)
BEGIN
  SELECT content INTO p_content
  FROM paste
  WHERE digest = p_digest;
END;
@@
```

```sh
varnishadm ban 'req.http.host == ptpb.pw && req.url == /'
```