Warn about using cookies for session storage

Possible exploits using malicious pickled objects are only possible
when cookies are used as storage backend.
Users should be explicitly warned about that and use the filesystem
storage instead - but they should also be aware of the cleanup they
will then need to perform manually.
Merge pull request #19 from moschlar/patch-1

Warn about using cookies for session storage
Merge branch 'master' of github.com:TurboGears/tg2docs