add code coverage analysis functionality to `qc'

Actually, I moved the code for setting up and calling `cover` to a separate module from `rebar_eunit` in order to be able to use it from `rebar_qc` as well.

- 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`