DISTINCT ON
I did a bit of digging and it looks like you can write:

```
SELECT DISTINCT(col1, col2), col1, ... FROM
```

So I'm going to close this since you can emulate this:

``` python
MyModel.select(fn.DISTINCT(MyModel.col1, MyModel.col2), MyModel.col1)...
```
 Actually it turns out the semantics are a bit different... reopening.
 Fixed by 19d2c2e738f874c

First-class support for this feature.
