Traversal implementations in-progress.
Implemented Pre-order traversal
in_order traversal kind of working
Implemented post order traversal with corresponding tests.
have in_order traversal and tests working
breadth first working with printing, but not as generator
have breadth first working as generator with no data structures
final cleanup and refactoring.  Two more tests written.  All pass.
Merged Perry's changes with mine for our BST traversal assignment.
removed unecessary _breadth_first method
corrected tests and bftraversal and in_order traversal to return value, not node.  made naming more consistent with sean's code.
resolving merge conflicts