Perspx/PXSourceList — pull request #41 by CrazyCatcher (2014-05-06 12:29:18+00:00)
Title: fix a Zeroing Weak References problem

When I use PXSourceList In `osx 10.7`, my program will crash exactly like [this problem](http://stackoverflow.com/questions/10904990/xmpp-objective-c-library-cannot-form-weak-reference-to-instance-0x7fcd4a49893) .

I make [a demo](https://github.com/CrazyCatcher/PXErrorDemo) for this problem.

Please see if that is this the right way to solved the problem.