Update the README.

I went through the setup instructions from the beginning with a fresh start. I noticed discrepancies and some missing steps. Reorganized a few sections, explicitly stated OSX installation instructions. I also added a basic 'contributing' section. Also, Chrome did not open for me when I ran Behat. Is another plugin required for that? I changed the Behat install instructions to /usr/local/bin instead of ~/bin because ~/bin is not in your PATH by default and didn't feel that making that assumption about folks was fair.