Added dict & functions for lesson 3

I still don't like the manual line endings, as it makes for a real hassle when making changes. I've been using sublimes word-wrap, although this means I have some rather long single lines. If you insist on them being 79 characters, you'll have to do that yourself! Also as I'm no markdown expert, please check through the syntax and make sure I've done this correctly. Also feel free to change as much of my stuff as you like - I hope it is reasonable to teach from and that I've not overcomplicated or missed stuff out.