Some updates/additions/bug fixes

- Changed tree structure to make it a little more conducive to testing.
- Added initial tests (by no means exhaustive)
- Fixed win condition bug where pressing enter with one letter left unguessed would automatically allow you to win
- Moved word list in to the constructor to make it easier to locate and update
- Moved main play loop in to its own method to assist with testing

Hope this helps you on your way, good luck!