config options and bug fixes

As promised on the Request Tracker Users mailing list, I'm sending you a pull request with a couple enhancements.  All changes are backwards-compatible; they will only take effect if someone sets values in `RT_SiteConfig.pm`.

* "+1d" and friends correctly increase the date by 24-hour increments when the user has RFC2822 date formatting.
* Stalling of tickets can be disabled.
* The top row of buttons can be hidden.
* The bottom buttons can be set from within `RT_SiteConfig.pm`.
* The "reset" button can be renamed/hidden.
* URLs are not broken when no operation occurs.

Please let me know if any of the commits worry you.  I'd be happy to split them out into separate branches if you prefer.

I made minimal changes to the `README` file, but I guess you would want to re-generate it now that I've extensively modified the documentation in `RT::Extension::StartsButtons`.