add new schema field 'showExcludedFromNav'

overrides method SitemapNavtreeStrategy.nodeFilter() in class NavtreeStrategy. The overriden nodeFilter returns True if 'showExcludedFromNav' also for nodes which are excluded from the navigation. The default is False. i.e. same as previous version
add 'showExcludedFromNav' to the Assignment
set showExcludedFromNav in the NavtreeStrategy

self.showExcludedFromNav = portlet.showExcludedFromNav
Merge pull request #38 from me-kell/master

Add field to show nodes excluded from Navigation