Document --unlock flag in deps.clean
Add number of skipped tests to ExUnit output

- Skipped tests now count toward the total
- Include number of skipped tests in runner return
value:

Before: %{failures: 0, total: 3}
After:  %{failures: 0, skipped: 1, total: 3}
Merge pull request #2898 from lstor/master

Add number of skipped tests to ExUnit output
Merge branch 'master' of https://github.com/elixir-lang/elixir