Add shopspring.Numeric

This adds PostgreSQL numeric mapping to and from
github.com/shopspring/decimal.

Makes pgtype.NullAssignTo public as external types need this functionality.

Begin extraction of pgtype testing functionality so it can easily be used by
external types.

float4_array.go

@@ -79,7 +79,7 @@ func (src *Float4Array) AssignTo(dst interface{}) error {
 			}
 		}
 	case Null:
-		return nullAssignTo(dst)
+		return NullAssignTo(dst)
 	}
 
 	return fmt.Errorf("cannot decode %v into %T", src, dst)