Improve tooltips

Allow tooltips in div element (currently tooltips only allow items in pre element which is tricky to format). Also fix wrapping, remove gradient fill, and introduce a maximum width of 500px by default.
I don't have a strong opinion, either about gradient or #FFFFEE style. Both do not look great, and both do not take into account the theme of the editor. (guess #FFFFEE is the right color for eclipse theme though).
I prefer to keep gradient, since it matches gutter tooltips in ace, and native tooltips on windows 7.
Setting default width for tooltips will interfere with autocompleters that provide formatted output. 

For the record, default css for .ace_tooltip is in [editor.css](https://github.com/ajaxorg/ace/blob/master/lib/ace/css/editor.css)