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 children plots in the wrong place
- convert selection index to screen data position? How to know what glyph to use?
- display tooltip only on one subplot at a time?

@kdodia any opinions or inputs welcome