Every generated file should include a URL for more information

It's important that anyone who comes across any generated file should be able to understand where it came from. imho, that means a URL reference.
- For an AutoValue generated implementation, the URL should be https://github.com/google/auto/tree/master/value.
- For a class generated by an AutoValue _extension_, the extension author should be compelled to supply this URL
- (others can comment re: AutoFactory and others)

Open questions:
- Should this URL simply be the content of @Generated? I think it should, and the current qualified class name we spit out is fairly useless. The fact that the @Generated annotation javadoc recommends FQCN as a convention matters basically not at all to me. The alternative is that we put this in a comment, which might as well be a javadoc comment (not that it seems remotely likely someone would be looking at generated javadoc for a generated class, but why not?)
- If the generators can even spit out a _version_-specific URL that seems even cooler....