Use Base64.urlsafe_encode64

This method complies with
"Base 64 Encoding with URL and Filename Safe Alphabet" in RFC 4648
see http://tools.ietf.org/html/rfc4648#page-7

Ran into problems when a Job would serialize to a Base64 string
containing a slash, since apache does not allow slashes in an URI:

```bash
/var/log/apache2/error.log
found %2f (encoded '/') in URI (decoded='/media/W1siZiIsInNhbGVzZm9yY2UvdXBsb2Fkcy9zb3VyY2VfZmlsZXMvMDA2TTAwMDAwMEFaRmxXSUFYLXVtbGF1dF/DpCAoMykuemlwIl1d/umlaut_\xc3\xa4.zip'), returning 404
```