Add test_doubly_linked.py with skeleton tests for instantiating expected classes and testing methods.
Add doubly_linked.py with skeletons for expected classes.
Flesh out skeletons for all tests.
Add structure to DoublyLInked() and Node() init methods
Update test_insert() and test_append() to verify the fully built list has the expected head/tail values.
Wrote insert method and revised test slightly to fit new nomenclature.
Wrote method for append, revised test.
added method for pop, slight test revision
added method shift
finished doubly linked list
Improve formatting and docstrings.
Add info to README about doubly linked lists