build: linking CoreFoundation framework for OSX

Even though libuv has no link time dependencies, linking CoreFoundation for OSX
is needed for OSX debugging features to function properly.

### Without linking CoreFoundation

We get a warning about certain heap debugging features not working.

![screen shot 2014-08-06 at 2 04 45 pm](https://cloud.githubusercontent.com/assets/192891/3831583/06b41f38-1d95-11e4-990e-6a4241fe7fd6.png)

### Linking CoreFoundation

When we link CoreFoundation everything works just fine.

![screen shot 2014-08-06 at 2 05 49 pm](https://cloud.githubusercontent.com/assets/192891/3831588/10a3e3d4-1d95-11e4-8e32-2732972cb999.png)