Moar PHP Exercises: Part 1

I changed up the `config.json` so that the exercises were in what I felt was a more realistic order (and matches exercism/xruby

Added: 
 * Raindrops
 * Robot Name
 * Roman Numerals
 * Difference Of Squares

I'll try and get a few more of these done over the next few days, but I thought I'd drop in today's efforts :)