adds removeHostListeners

[stardisblue]

Gives the ability de remove all the event listeners, this should only be used when throwing away the EasyPZ instance. *(see note)

Event functions (e.g.: onMouseUp) were changed to preserve the this binding, that way, they can be passed directly in addEventListener and removeEventListener.

note :

We could set setupHostListenersto public to allow EasyPZ to reattach. But EasyPZ preserves transformations, so it might create undesired side-effects if the transformation was affected by another party.

Maybe I will put another pull request to reset all EasyPZ tranformations, which might satisfy a lot of cases.

Other :
Equivalent working example : https://observablehq.com/@stardisblue/hello-easypz

fixes #13

[michaschwab]

Thanks for your contribution! I didn't like turning the methods into anonymous functions, so I changed it slightly to instead use bound functions before assigning and removing the listeners, but no substantial change.
Can you please take a look to see if it still works the way you want after my changes? If so, I'll merge your PR!

[stardisblue]

My method was at first glance less verbose, but i found that it causes problems when extending (ref: https://stackoverflow.com/a/45882417).
I don't see any problems with this method. 🎉