Fixing issue #154

Issue #154 reported the following error

    undefined method html_options for SimpleNavigation::ItemAdapter

when calling `render_navigation` with the `:items` option. This was
caused by the absence of conversion from ItemAdapter to Item in
ItemContainer and was introduced in commit 12bf8a6d.
Merge pull request #155 from simonc/154-html-options-on-item-adapter

Fixing issue #154