Adds foldTree function

I get the gist of using `foldr` to convert a list into a binary tree (I used it to build the `MessageTree` in `LogAnalysis.hs`), but I had trouble implementing an `insert` function so that the tree always remains balanced. I looked at [Build balanced binary tree with foldr](http://stackoverflow.com/questions/16157203/build-balanced-binary-tree-with-foldr) on Stack Overflow for guidance. I'm not wild about all the `where` clauses, but it seems to work. I'll see if I can refactor it after a little more thought.
Updates README.md

Adds links to several repositories with alternative solutions to the exercises from UPenn's CIS 194. Always cool to see different solutions for the same problems. You never know what you'll learn from looking at someone else's code until you have!