Fix for Issue #92

What : Fix for https://github.com/coopernurse/gorp/issues/92

Fixes :  Added new UpdateWithSelector method for controlling which columns are included in generated update statement.

```
type Person struct {
		FirstName     string `db:"first_name"`
		LastName     string `db:"last_name"`
		Email     	  string `db:"email"`
		Phone     	  string `db:"phone"`
	}

coll = []string{"first_name", "last_name"}
f := func(cm *gorp.ColumnMap) bool{
	for _, a := range coll {
		if a == cm.ColumnName {
			return true;
		}
	}
	return false
}

count, err := trans.UpdateWithSelector(f, &person)

```