Application-related uris

Not sure about asp.net mvc, but in asp.net (as under mvc we still have asp.net engine) for site root we might use HttpRequest.ApplicationPath (additional transformations might be needed)

Maybe a ${SiteRoot} could be used in both Monorail and asp.net mvc. Then a set of attributes is not needed to be known in advance.

Added a node visitor which looks for "~/" at the start of a number of known attributes. It's essentially changed to "${SiteRoot?}/" which is a property that's now available to mr and asp.net mvc.

The asp.net mvc siteroot is normalized as follows

private string _siteRoot;
public string SiteRoot
{
    get
    {
        if (_siteRoot == null)
        {
            var appPath = ViewContext.HttpContext.Request.ApplicationPath;
            if (string.IsNullOrEmpty(appPath) || string.Equals(appPath, "/"))
            {
                _siteRoot = string.Empty;
            }
            else
            {
                _siteRoot = "/" + appPath.Trim('/');
            }
        }
        return _siteRoot;
    }
}

The following attributes are examined. These are the attributes that were of type URI in the html spec.

_specs = new[] 
{
    new ElementSpecs("a","href"),
    new ElementSpecs("applet", "codebase"),
    new ElementSpecs("area", "href"),
    new ElementSpecs("base", "href"),
    new ElementSpecs("blockquote", "cite"),
    new ElementSpecs("body", "background"),
    new ElementSpecs("del","cite"),
    new ElementSpecs("form", "action"),
    new ElementSpecs("frame", "longdesc", "src"),
    new ElementSpecs("head", "profile"),
    new ElementSpecs("iframe", "longdesc", "src"),
    new ElementSpecs("img", "longdesc", "src", "usemap"),
    new ElementSpecs("input", "src", "usemap"),
    new ElementSpecs("ins", "cite"),
    new ElementSpecs("link","href"),
    new ElementSpecs("object", "classid", "codebase", "data", "usemap"),
    new ElementSpecs("script", "src"),
    new ElementSpecs("q", "cite") 
};