Feature/toolbar enhancements

resolves https://github.com/ContinuumIO/bokeh/issues/1156, https://github.com/ContinuumIO/bokeh/issues/1157, and adds Tooltip Help icon
![hover-help](https://cloud.githubusercontent.com/assets/6355636/4193553/b97c411e-37a2-11e4-9775-24aabada126b.png)