Allow nil receivers

For example:

var p *Person // <nil>
dmap.SelectOne(&p, query)

If nothing was found, the receiver is still <nil>

This is supported in other mappers like mgo because it requires less typing if
you need a pointer, and it's also easier to check if nothing was found by
simply comparing the receiver to nil afterwards:

if p == nil {
  error("User was not found!")
}