B tree in master (sans removing individual nodes) - ready to merge

Hi mate

I've completed insertion, lookup, and complete tree deletion for B-trees. Insertion and lookups are all covered by tests quite extensively and I am happy that they are production ready.

I've merged B-tree branch into master (after syncing with your master first of course) despite not being able to remove individual nodes yet. I think the structure can be useful even in this state if someone's gonna use it for quick lookups.

I also noticed that you've broken linked list in master so nothing builds at the mo.

I might make a start on removing a record later today, will see how I feel.