tests now simulate ruby platform and commands being present
or missing on the system.

Less juggling of constants, more ruby methods. Information that requires
shelling out (e.g. GitHub username, token, repo name ...) is now loaded
on demand, meaning no shelling out for commands that don't need it.

Tests are now safer; they are prevented from shelling out to get info.
All test data is stubbed.

New `github_url` method builds needed URLs while keeping sensible defaults.

Commands that open the browser (`compare`, `browse`) now share a bit of
underlying implementation and recognize the same flags (`-p`, `-u`).

Fixed a problem in the ronn file this caused.

Usage example:

  $ git fork
  (forks "defunkt/hub" on GitHub)
  (git remote add mislav git@github.com:mislav/hub.git)
  new remote: mislav

Use `git fork --no-remote` to skip adding the new remote.

Tests use Webmock for stubbing out HTTP requests:

  $ gem install webmock

useful for jumping to the GitHub page of the repo checked out in the current directory

most of the time it's `git`, but in some cases it can be a different
executable like `open` for `hub browse` or `echo` in testing