Update search classes to work with new search system

Add new properties to pass to search results (for future use), re-add the 'text' property so that client JS will show the extra description line, and switch to the new ownCloud namespace scheme.
Rename results file
Add 'search by date string' functionality

Per @libasys' request, added the ability to search calendar events by date/time; this uses PHP's strtotime() so it should be able to interpret even some natural-language queries like 'today' or 'next Sunday' (see http://www.php.net/manual/en/datetime.formats.relative.php)