OSX backend. 2D histograms are flipped vertically

This bug only exists on master but not in 1.4.2. Moreover this only happens on screen but is correct when saving the figure. I guess this relates to how the y-axis is inverted in images. 

This can be seen by running api/power_norm_demo.py where the covariance is inverted. 

The qt4/5 gtk3Agg and tkAgg are not affected.

OS x (screenshot)

![screen shot 2014-11-16 at 17 55 25](https://cloud.githubusercontent.com/assets/548266/5062430/c576e2e2-6db9-11e4-8206-390ee1113c7a.png)

OSX result (saved fig)
![figure_1](https://cloud.githubusercontent.com/assets/548266/5062423/5be6d9f4-6db9-11e4-9e59-02724f111eee.png)

In the docs:

![power_norm_demo](https://cloud.githubusercontent.com/assets/548266/5062426/82750e06-6db9-11e4-9458-e9a64fea9c22.png)