Code cleanup and added flexibility

Notes:
* I couldn't get the yaml formatting right in the readme, so I used a markdown cheat.
* Replaced a lot of repeated filenames with variables at the top
* I'm not based in the UK, so the URL wasn't working for me.  I moved the domain into the yaml, so that the same script would work internationally.
* My system (OS X) didn't have a notify-send command. I put in a method that would check for the command, and skip the notify-send stuff if the command is missing.  This is a bit of a hack, but I didn't need this functionality.
* I switched File.exists to File.readable, since a file could exist without being readable and the script would still be sad.