Add endpoint for logout

Hitting /oauth2callback/logout will now invalidate the sproxy cookie.
Allow redirects after logout by specifying the state query parameter
Whitespace change
Update README

The cookie is not called `gauth`, the actual name is configurable.  In
the README we now refer to it as "a session cookie" (emphasizing
*session cookie* when we first mention it).
Add documentation for logout endpoint to README
Add specs for logout endpoint
Move OAuth callback and logout endpoints under /sproxy/*
Add logout page (see #3)
DRY up cookie construction