Add number of skipped tests to ExUnit output

Based on [discussion on elixir-lang-talk](https://groups.google.com/forum/#!topic/elixir-lang-talk/5lZnUIO3YDw). When running ExUnit, it shows the number of skipped tests:

    Finished in 4.8 seconds (3.1s on load, 1.7s on tests)
    138 tests, 0 failures, 1 skipped

Since this output is not tested in any current unit tests, I decided not to write any new tests. Let me know if you want me to write automated tests as well.

I couldn't find any places in the documentation that need updating because of this change, but I might have missed something.

Currently, `ExUnit.run/0` returns the number of tests and number of failures. **Should I update it to return all three numbers?**