HstSiteMapItem should have a getScheme() method to expose the hst:scheme property of the hst:sitemapitem node.

The HstSiteMapItem should have a getScheme() method to expose the hst:scheme property of the hst:sitemapitem node.
It will return the following:

• if the property is set on the site map item itself, it uses that
• else it uses the parent site map item's scheme if it has a parent
• else it uses the mount's scheme.

if the hst:scheme property is set, but it is left blank, the site map item will default to the default scheme for virtual hosts (currently 'http').

This will in turn allow linking to create better links to secure site map items and will also allow an easy hstsitemapitemhandler to redirect to https when needed.

This issue partly solves HSTTWO-1303.