Fix warnings building with -Warc-bridge-casts-disallowed-in-nonarc

In Xcode 4.6.3, building with iOS 6.1 as a target and using `-Warc-bridge-casts-disallowed-in-nonarc`, I get several warnings in `BKMacros.h`: `'__bridge' casts have no effect when not using ARC`.

This pull request just removes those `__bridge` casts so the project builds properly.

I haven't tested this on OSX so not sure if there are any other side effects, but it would be nice to build on iOS with stricter settings. Thanks for a great project!