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?

**TODOs**

- [x] Only show number of skipped tests when there are any.
- [x] Return `skipped` tests from `ExUnit.run/0`.
- [x] Update tests to verify added output.
- [x] Update tests to verify return value of `ExUnit.run/0`.
- [x] Update docs for `ExUnit.run/0`.
Completely agree, I think the current way is counter-intuitive. That might be apparent from my changes here :)