Extract authenticators to their own files

[nevans]

I've started down the path of implementing #10. This PR simply copies authenticators with some minor changes, but this raises a big issue I've been meaning to ask about: SASL. I'll open another issue to discuss that: #23. I'm marking this as a draft, pending that discussion.

In addition to moving the authenticators to new files, I've updated the rdoc and added authnid support for PLAIN.

[shugo]

It looks good to me.
@hsbt Do you have any considerations in terms of Ruby releases?

[hsbt]

👍 It seems good practice for organizing code.

@nevans Can you also move net-imap.gemspec to under the net/imap dir?

[nevans]

@nevans Can you also move net-imap.gemspec to under the net/imap dir?

If we move the gemspec, won't that break existing build scripts? In particular, it means that people won't be able to use the common pattern: gem 'net-imap', github: "ruby/net-imap", ref: .... They could add glob: 'lib/net/imap/*.gemspec' but that's a fairly uncommon option, especially when there's only one gem in the repo.

[nevans]

And... I only just discovered that bundler's default glob is "{,*,*/*}.gemspec". I'd assumed it was different, matching less. Still, that means the gemfile would need to be in the repo root or one dir down. Neither lib/net/imap/net-imap.gemspec nor net/imap/net-imap.gemspec would work. Am I misunderstanding something? 🙂

[hsbt]

f we move the gemspec, won't that break existing build scripts?

Ah, Sorry. I meant it in ruby/ruby repo. There is no action with this repo.

[hsbt]

I tweaked sync script at ruby/ruby@5de6f1a