Improve error message when user attempts to select into a non-pointer slice.

If user passes a type of []*Struct instead of *[]*Struct, tell them so.

Tested by changing this line in gorp_test.go:

       var puArr []*PersistentUser
       _, err = dbmap.Select(&puArr, "select * from PersistentUser")

to

	var puArr []*PersistentUser
	_, err = dbmap.Select(puArr, "select * from PersistentUser")

and observing this error message on test failure:

	panic: gorp: Cannot SELECT into a non-pointer slice: []*gorp_test.PersistentUser