Allow multiple --css options
Make restview --long-description autoreload, add --watch

You can use -w with --long-description, to add additional files to watch
if your setup.py doesn't read the default files.  For example if you use
NEWS.rst instead of CHANGES.rst, you can do

    restview --long-description -w NEWS.rst
Make --watch work with regular files too

Make our JavaScript reload stylesheets as well.

This enables incremental stylesheet development with

    restview --css my.css -w my.css sample.rst
Remove old styles correctly
Paranoia: remove old styles _extra_ correctly

I guess I'm afraid that some DOM implementation might return a list that
doesn't reflect DOM changes, which would make the while() loop forever.
Increase mtime precision

The JS side deals with it as an opaque cookie, without doing type
conversions.  As long as we can rely on Python to convert floats to
strings deterministically, we're ok.
Mention the new --watch option in the changelog
Reload error pages too
Unify error style with restview.css
Improve error message for process errors
Show the filename in error messages

(Instead of "<string>:123")