Add solutions from Dave and Leo

Some solutions for MIT problems from myself and Leo for 1-7 (we were working on 8). These are also in https://github.com/actionshrimp/sicp-exercises which contains some solutions for other weeks I've attended too. 

I will attempt to split the rkt file up into a more file-per-problem structure at some point. Also we have some more written answers on a bit of paper which need typing up, but I managed to type up a couple.