Add support for ordering by list of IDs
If the list of ids is ordered, then you can simply do:

``` python

MyModel.select().where(MyModel.id << list_of_ids).order_by(MyModel.id)
```

If the ordering is arbitrary, then you could write a helper like this:

``` python

def sort_by_id(query, id_list):
    # create a mapping
    mapping = dict((model.id, model) for model in query)

    for id in id_list:
        yield mapping[id]
```
 Thanks for this. I found dirty but DB driven solution that works in MySQL:

```
list_of_ids = [1,2,3,4,5,6]
result = MyModel.select().where(MyModel.id << list_of_ids).order_by(R("FIELD(t1.id, %s)" % ", ".join(map(str, list_of_ids))))
```
 interesting.  another way you can do that is:

``` python

MyModel.select().where(MyModel.id << list_of_ids).order_by(
    fn.Field(MyModel.id, *[R(str(id)) for id in list_of_ids])
)
```

Would by nice if this worked:

```
list_of_ids = [1,2,3,4,5,6]
result = MyModel.select().where(MyModel.id << list_of_ids)
# result is now is same order as list_of_ids
```
