Use `stargazers` (instead of `watchers`) to sort forks

In API v3, for backwards compatibility purposes, watchers and stargazers are the
same thing. [1]

API v3 now supports `stargazers` as an option for sorting the list of forks for
a repo. API v3 will continue to support `watchers` as an option for sorting,
but we're no longer advertising that support. It does the same thing as
`stargazers`. Any new API client code should use `stargazers` instead of
`watchers`.

[1]: http://developer.github.com/changes/2012-9-5-watcher-api/
Tidy up description of sort parameter
Add `stargazers_count` to summary repository representation

In API v3, for backwards compatibility purposes, watchers and stargazers are the
same thing. [1] So the watchers_count is **really** the number of stargazers.
Let's add a stargazers_count attribute as one small step toward helping API
consumers figure out how to get the number of stargazers for a repo.

[1]: http://developer.github.com/changes/2012-9-5-watcher-api/
Add `subscribers_count` to detailed repository representation

When you fetch the full details for an individual repository, the JSON response
will now include the `subscribers_count` attribute.
Fix busted link
Update plans for watchers/stargazers/subscribers methods

- Strikethrough phases 2 and 3. They will not be part of API v3.
- Provide a "UPDATE" section describing this change to the roadmap. (Good news!
  You don't need to change your API client code!!)
Expand explanation re: stargazers/watchers/subscribers endpoints