Allow default boundary and capacity about quadtree

[Dezzles]

I'm happy to take a look at this
Just a thought, would it make sense to have a static createDefault function that has this behaviour? That way this would be a non-breaking change

[shiffman]

@Dezzles Can you explain more? (Sounds like a good idea but I'm not sure I understand completely.)

[Dezzles]

Instead of new QuadTree() creating a tree with default values, using something like
QuadTree.withDefaults()
That would invoke new default values
It would keep the constructor concise and predictable which I think is important

[Dezzles]

Quickly threw this together over lunch and put it in PR #37
Take a look and let me know what you think?

[crhallberg]

Closing this issue since #37 resolves this issue