Fix hover tooltip going off page

Closes https://github.com/ContinuumIO/bokeh/issues/675

This could benefit from a few pixel-tweaks; @bryevdv @damianavila please review.

![fullscreen_6_5_14__7_45_pm](https://cloud.githubusercontent.com/assets/4944634/3195575/0473c5f4-ed14-11e3-9fdd-e27c4789f2ae.png)