When using @skip decorator, `reason` argument is required. But one could easily overlook that and use it like so:

    class SomeTest(TestCase):

        @skip
        def test_method(self):
            # ...

The test actually passes when running with unittest, because it's equivalent to

    class SomeTest(TestCase):

        def test_method(self):
            # ...
        test_method = skip(test_method)

This gives the illusion that @skip decorator worked as expected.

I propose we should check the passed in `reason`, and raise error if it looks like a function/method, therefore prevent this erroneous usage.

In practice, this behavior has misled people using the decorator in the unintended way. For example: In this chapter of Test-Driven Development with Python (http://www.obeythetestinggoat.com/book/chapter_organising_test_files.html), the author used @skip decorator without `reason` argument.