Pass a map with refs to store columns to MapScan()

Currently, as far as unmarshal concerned, ``MapScan()`` is not as flexible as ``Scan()``.
Unlike ``Scan()``, ``MapScan()`` can not handle ``Unmarshaler`` or any other smart unmarshal logics gocql equips.
It is because ``MapScan()`` can't receive arbitrary typed variables which you want to store columns into.
Actually, this limitation has bothered me fo a long time. I would like to use ``UnmarshalerCQL()``.

In this patch, I make ``MapScan(map[string]interface{})`` receive an map which contains reference variables in its elements. If a column name already has the ref-value, ``MapScan()`` set the value to the reference.
By this extension, ``MapScan()`` can handle any type conversion as well as ``Scan()``.

Could you review this extension?

for example:
```go
type FullName struct {
	FirstName string
	LastName  string
}
func (n *FullName) UnmarshalCQL(info *TypeInfo, data []byte) error {
	t := strings.SplitN(string(data), " ", 2)
	n.FirstName, n.LastName = t[0], t[1]
	return nil
}

var age int
var fullname FullName
// Set some references to this map before passes it to MapScan()
m := map[string]interface{} {
  "age": &age,
  "full_name": &fullname,
}
err := session.Query(`SELECT id,full_name,age,weight  FROM users limit where id='x'`).Iter().MapScan(m)
fmt.Printf("ID:%v, AGE:%v, FN:%v LN:%v W:%v¥n",
  m["id"],
  m["age"],
  m["full_name"].(FullName).FirstName,
  m["full_name"].(FullName).LastName,
  m["weight"]
)
```