Support Postgresql array types

https://github.com/lib/pq/issues/49

This PR has support for:
- scanning text[], varchar[], int*[], float*[]
- valuing text[], varchar[], int*[], float*[] **in statement.Exec/Query/QueryRow only**

Known limitation: slice type cannot be converted if used directly by db.Exec/Query/QueryRow
Not being able to distinguish between an empty string and a NULL in an array element is not going to fly.  Trying to scan an array with NULL elements into a []string ought to return an error.