include libkern/OSAtomic if available

I added [rake-compile](https://github.com/luislavena/rake-compiler) to manage compilation on the C side (it can do the Java side too, I just haven't figured it out yet).  Also, I couldn't get the extension to compile because it wasn't including libkern/OSAtomic.h, so I fixed that.  Also test/unit wasn't working with trunk ruby, so I switched it to minitest.