adding Mac installation instructions

Hey,

I'm in the process to add installations instructions for the Mac. I can't get it work though, I could use some help.

It fails at `easy_install --no-deps packaging/pkgs/Fiona-0.7.tar.gz`

```sh
$ easy_install --no-deps packaging/pkgs/Fiona-0.7.tar.gz
Processing Fiona-0.7.tar.gz
Writing /var/folders/7k/w2f6gw9n60g_d_7q248lg3h00000gn/T/easy_install-pPdjsr/Fiona-0.7/setup.cfg
Running Fiona-0.7/setup.py -q bdist_egg --dist-dir /var/folders/7k/w2f6gw9n60g_d_7q248lg3h00000gn/T/easy_install-pPdjsr/Fiona-0.7/egg-dist-tmp-GzI_Sr
ERROR:root:Failed to get options via gdal-config: [Errno 2] No such file or directory
src/fiona/ogrinit.c:235:10: fatal error: 'cpl_error.h' file not found
#include "cpl_error.h"
         ^
1 error generated.
error: Setup script exited with error: command 'cc' failed with exit status 1
```

Any idea?