MatrixBase.is_zero supporting None results.

Extended original implementation of MatrixBase.is_zero to return None if the
matrix may or may not be a zero matrix.  Also found a workaround to issue 4114
to have this method accessible to ImmutableMatrix.

See http://code.google.com/p/sympy/issues/detail?id=4114
Merge pull request #2637 from Tempel/matrix_is_zero

MatrixBase.is_zero supporting None results.