Fix stacklayout size hint

Example of problem: green is StackLayout size, red is Child size.

![layoutfix](https://cloud.githubusercontent.com/assets/1936976/4963323/c305e956-6709-11e4-88db-ac2df9add0e4.png)

This solution is only accurate for stacklayouts that go in one direction. Our layout algorithm is totally incorrect for a variety of other potential orientations as is, this at least fixes sizing for the simple case. StackLayout will still often place a widget outside of its bounds instead of properly slotting it in. This occurs if you try to do an arrangement such as widgets with size_hints 1: (.5, 1.0), 2: (.5, .5), 3: (.5, .5) which should look like:
12
13
if arranged lr-tb, but often ends up:
123
1