Fix typo in README

Ur/Web generates excutables with an .exe extension. The README
left this out.
Use relative path in urweb_persona.js script inclusion
Change Identifier to not be an option type

Addresses comment from Adam Chlipala on mailing list
Change uneeded 'return' in signal code

Addresses review comment by Adam Chlipala on mailing list.
Refactor status signal to use user id

This changes the status signal to be the actual user id
rather than an HTML fragment. This allows dyn blocks to
monitor changes in the username of the logged in user.