Christophe de Vienne <cdevienne@gmail.com> [Wed, 11 Dec 2013 17:52:54 +0100] rev 9347
[rset] New method: ResultSet.one()
This method will return exactly one entity while enforcing its existence and unicity.
The idea is shamelessly borowed from SQLAlchemy Query.one().
Closes #3352314
[jcr: use len(self) instead of len(self.rows)]