Unbind events

This addresses #38 by ensuring that events are properly unbound on shutdown. 

I updated the many-series example page to better show the before/after results. 
Load the page, perform a heap snapshot in Chrome. Click the replot link, then perform a second heap snapshot. 
Finally, click again, perform a third heap snapshot. Select the third snapshot, and select items created between snapshots 1 and 2. 
Before the second commit, you will see that the plot tooltip plot hover event handlers remain, leaking memory.