BarButton items not rendered at Retina resolution

Due to use of UIGraphicsBeginImageContext and not UIGraphicsBeginImageContextWithOptions, BarButton borders etc were rendered at non-retina resolution, so the edges looked all blurry. Fixed with this pull request.

Before:
![Screen Shot 2013-01-09 at 12 17 13](https://f.cloud.github.com/assets/288569/53261/9abe3dec-5a57-11e2-9be2-3c050e00a54d.png)

After:
![Screen Shot 2013-01-09 at 12 20 00](https://f.cloud.github.com/assets/288569/53262/a6ab7c6e-5a57-11e2-8e58-982facaa4107.png)