Eliminated requirement to use -all_load linker flag.

Hi guys,

Thanks so much for this awesome library, our team really enjoys using it. 

When we updated our pod dependency from 1.5.3 to 1.5.5, our build broke during linking. I've been able to pinpoint the issue to the -all_load linker flag. There is a known bug with the linker when bringing in categories: http://developer.apple.com/library/mac/#qa/qa2006/qa1490.html

I also found a blog post with more descriptions on the issue:
http://webcache.googleusercontent.com/search?q=cache:FQvKOYmv6mAJ:blog.binaryfinery.com/universal-static-library-problem-in-iphone-sd+&cd=1&hl=en&ct=clnk&gl=us

The blog references a fix RestKit used:
https://github.com/RestKit/RestKit/commit/b96940cc6427381e8b4afce47aa9f30667314a14

This pull request adds a new macro for embedding a dummy class in all the category implementation files so that the linker can function without the -all_load flag. I built and ran tests for both Mac and iOS.

Looking forward to your feedback!

Cheers,
Rene