$.helptip improvements

If no option.content is provided, it tries to get the title attribute or the text inside any span with class _ht_tip inside the element
If helptip content contains a link, it will delay hiding the helptip to enable user to hover out of the element to click the link inside the tip
Finally, the plugin was implemented incorrectly:
1. $(this) could possibly be a list of elements thus $(this).hasClass('_ht_track') and $(this).hasClass('_ht_delay') could return wrong information. This is moved inside map() so that has_class is called on individual element.
2. The map's callback should not return $(this) as then map will return an array of jquery objects instead of returning one jquery object with all elements in it.
Explicit delay not needed anymore

The plugin handles the delay itself.
Allow HTML in column header's helptips
Helptip for column headers can now handle html.