you don't need to implement Execer and Queryer

Not sure if this is worth a PR, but you don't need to implement these. They're implemented in database/sql. Effectively, that code in conn.go will never be hit. I tweaked the unit test to check for Execer--it wasn't really testing anything before. 

Exec:
http://golang.org/src/pkg/database/sql/sql.go?s=22844:22913#L836

Query:
http://golang.org/src/pkg/database/sql/sql.go?s=23894:23963#L885

Also, apparently my `go fmt` autorun when exit caught a few things.