MatrixBase.is_zero_matrix supporting None results.

Extended original implementation of MatrixBase.is_zero to return None if the
matrix may or may not be a zero matrix.  And, as a workaround to issue 4114,
this was renamed to is_matrix_zero so that it would be available to
ImmutableMatrix.  The original method name, is_zero, is set as an alias to
is_zero_matrix for backwards compatibility, but references to it have been
replaced with the new method name to ensure everything works correctly with
ImmutableMatrix.

See http://code.google.com/p/sympy/issues/detail?id=4114