Document that the dependency marker also works if applied to a class as a whole

[RKrahl]

@anishradvani requested in #38 that it should be possible to apply the dependency marker on a test class rather then to an individual method. Actually, it turns out that this already works. Consider:

import pytest


@pytest.mark.dependency()
@pytest.mark.xfail(reason="deliberate fail")
def test_f():
    assert False


@pytest.mark.dependency(depends=["test_f"])
class TestClass(object):

    def test_a(self):
        pass

    @pytest.mark.dependency()
    def test_b(self):
        pass

    def test_c(self):
        pass

This will have the effect that test_a and test_c will be skipped, because they depend on test_f, while test_b will be run.