added classifiers

probably incomplete, but better than nothing
redesigned page layout

based on mock-up:
http://fnd.github.io/bfw-mockup/

no navigation links yet
clarified user handling

... by renaming variables - prompted by a minor efficiency enhancement
(avoiding repeated references to `environ['tiddlyweb.usersign']`)
moved form decorator into separate module
started integrating nav links

front page and dashboard only for now

note that this, as well as the mock-up* revealed that we don't want to
redirect auth'd users from the front page to the dashboard anymore (this
change has not been implemented yet though)

* http://fnd.github.io/bfw-mockup/