Fix issue #879: series of undefined functions.
Issue 879: add test for series of a subclass of Function.

Now we can use also

    >>> class TestF(Function):
    ...     pass
    >>> TestF(x).series(x)
Issue 879: fix python3 failure.
Merge branch 'master' into pull1888

Conflicts:
	sympy/core/function.py
Merge pull request #2224 from tswast/pull1888

Pull1888