iOS 7 implementation of `rightTriangleImage`

Updated implementation of `- (UIImage *)rightTriangleImage` to
properly rotate image. Previous method does not work in iOS 7.

Before:
![bad](https://f.cloud.github.com/assets/15388/1318523/3b1fb5a6-32be-11e3-9e10-d755f3e9ea87.png)

After:
![good](https://f.cloud.github.com/assets/15388/1318524/407a1bcc-32be-11e3-9805-787e33af06d7.png)