Add code coverage analysis functionality to `qc'

- Use `cover' with QuickCheck testing
- Reuse the `cover_*' rebar.config options
- Refactor cover-related code to separate module (`qc_cover_utils')
  for use with both `eunit' and `qc'
Merge branch 'horvand-cover-for-qc' into next
Merge branch 'erlc-speedup-v5-fixup' into next
Merge branch 'missing-help' into next