Added a banner to the beta site

Just as a heads up for people who mentally or actually filter phpweb
SVN commits: I've just made a commit that adds a banner to the top of
the beta site explaining that it's a work in progress, could change at
any moment, may cause imminent death, is probably broken, and that
contents may settle during transit.

The beta's getting some attention on Twitter and the like, so I felt
it would be best to set the expectations up front that it's not the
finished product, because some people are assuming that it is. The
close button on the bar sets a cookie that prevents it from recurring
for 30 days, so it shouldn't be too annoying for people actually
trying to use it as their main site for testing purposes. (Or because
they think it's pretty, like I do.)

My apologies for the added ugliness and potential clunky wording.
Commits to fix one or both welcome. :)

Adam