Teach driver how to handle uint64 by implementing ColumnConverter.

[giulioungaretti]

This allows one to actually store and retrieve uint64 values stored as numeric in postgres.

The idea is taken from the default database/sql (https://golang.org/src/database/sql/driver/types.go#L213) .

I would like to write some tests for it but running any test (with master branch) just returns this :

--- FAIL: TestPgpass (0.00s)       conn_test.go:163: , unexpected error <nil> 

[arp242]

This was added in #1125 and #1238 with CheckNamedValue(), which does the same as the (now-deprecated) ColumnConverter().