Change to C code to correct the 'empty' broadcasting issue.

einsum.c.src - in prepare_op_axes() change so it only uses the
old BROADCAST_RIGHT case.  This removes to 'cannot broadcast None' error
test.  Also the BROADCAST_LEFT case is not needed.  RIGHT does just as
well.

Other changes to this file are from the newer master

test_einsum_c.py - test file direct from the master

einsum_py.py - modify some test cases to exercise the changes np.einsum
Also in the middle of refining how arguments are being passed around.