Modifications to DFSIterator. Hopefully next is now working.
Initial tests for DFSIterator.
Renamed DFSIteratorTest to generic IteratorTest.
Added tests for expected orders in traversals.
Revised iterator for Python3 style (next to __next__).

However, DFSIterator test is not yet terminating. :C
Added debugging prints. BinaryTree.__str__ is currently for debugging.
More debugging code.
Found out what's wrong with the debugging prints.

Wrote my own debugging print function to remedy this.