Add basic HTML/JavaScript to enable authentication

This should be accessible using `raw.github.com/...', but if not, I
will put it on my webserver.  For now, this seems like it's as good as
it's going to get.

When the page is accessed with an access token, this token is
displayed (styled as CSS class `uservalue', big, dark, and green).

Error conditions are not currently being handled, but they will be.
Implement OAuth2 authentication

Tested with commit 1749aa32c25d8eee9ca0753cb55bd7d134f320b7, current
HEAD of the `gh-pages' branch.
Further explain `stack-cache-directory'

I looked at it for a few minutes and wondered how to use it -- and I
wrote the thing!  :)
Remove files duplicated in `gh-pages'

These pages should be tracked by that branch; it is an explicitly
separate part of the system.
Use standard '.el' extenstion for cache

This should probably be altered so that the caching function requires
symbols; this would keep everything consistent since the caching
function could apply the same pattern to every symbol it receives.
Prune unnecessary variable

This key is actually used in request-making on the whole, so it has been
moved to `stack-core.el' as of 9cb376361983742f0a15775b233fc194d61b3ea7.
Change constant to variable

As it is let-bound in the authentication flow, it is not truly a
constant.  Add a warning to the documentation appropriately.
Remove unused permission

We are not using `private_info' yet, so there is no need to request this
permission.  It can, of course, be easily added later on if the
information is ever usable.
Merge branch 'master' into auth
Merge branch 'master' into auth

Conflicts:
	stack-core.el
Merge pull request #25 from vermiculus/auth

Authentication