Implemented wjd's suggestions and added test code

Note: some tests are failing, so code still needs some debugging.
Fixed two of the three failed tests.

Two of the three test failures were caused by bugs in the testing code.
Only failure of Test 1 (get_length) remains to be fixed.
Fixed wjd's stupid bug in get_length function.

The function was returning len(s) instead of len(dna)
because s was the variable name I used in the first
version of this function that I wrote earlier.
initial commit of README.md