Add favicons to vomnibar completions

Favicons are displayed as a background-image style next to urls in
similar style to chrome's history page.
For tabs the icon is fetched from the url specified by chrome tab's
favIconUrl property. Unfortunately extensions to not have access to
chrome://favicons/ to get bookmark favicons but fortunately most sites
serve a favicon from protocol://hostname/favicon.ico
(e.g. http://example.com/favicon.ico) so this url is tried for non-tab
types.
Merge pull request #1197 from smblott-github/command-option-tests

Add tests to validate command structures.
Reduce max removeTab to chrome.session.MAX_SESSION_RESULTS.
Merge favicons for vomnibar.
Fix tests for favicons.
Validate each advanced command is in a command group.
Ensure advanced commands are in command groups.