Restructure README.md

I was just taking a look at your talk for tonight and came across this repo. I hope you don't mind, but I took the liberty of restructuring the README in order to make it a little easier to follow. Feel free to close this without comment if it's not really of any use to you.

**Changes:**
- Add header images
- Add table of contents
- Restructure outline
- Add syntax highlighting to PHP examples