New interface for clean, allowing more control

- Now, when clean is passed no arguments, it simply returns the set of
  packages which are safe to clean
- Calling 'clean' with arguments attempts to remove those packages
Modify interface; add 'unused_packages' function
Missing lib.loc in status call
Reintroduce dry.run to clean
Update documentation; dontrun{} examples
Merge pull request #90 from rstudio/feature/clean-new-interface

New interface for clean, allowing more control