fix gridplots with hover tools

fixes #598 

@kdodia can you confirm this fixes the proximate problem and allows grid plots to display with a hover tool attached?

There are still issues to resolve, probably not all in this PR:
* what to do when there are multiple "hits"
 - show tooltip only for nearest
 - show multiple tooltips
* "secondary" tooltips in other subplots in the wrong place (same screen coords as originating plot)
 - convert selection index to screen data position? How to know what glyph/data to use?
 - display tooltip only on one subplot at a time?

@kdodia any opinions or inputs welcome.