This commit is not precisely atomic, but the changes have evolved, summary:
* The ENV-based interface has been moved upwards, the generator has now a conventional
initializer.
* RAILS_VERSION is now assumed to be a Git tag. A blank RAILS_VERSION means edge guides.
* In consequence, the EDGE env variable is gone.
* The "local" version is also gone, the current SHA1 is computed for edge guides.
* Assumes guides are generated from a repo checkout (time ago users could
generate them from gems.)
* The WARNINGS flag is gone in consequence, you cannot disable warnings.
* The `api_link` Markdown helper is fixed.
* Docs about usage have one single place: rake guides:help.
* Links in guides have been revised.
