merge

Thank you. I am very glad to create pull request. I modified to meet my application, 
Maybe they are not suitable for everyone.
First, I did not understand why the "key pointer" in node_t structure rather than point_t structure. For ease of use, I moved it to the point_t structure.
Second, I modified quadtree_search and quadtree_insert interface, making the point pointer return and adding the "update" parameter in the insert operation.
Third, there may be a memory leak in the insert operation, I fixed it.
Now,  in the update insert operation, there is still a small bug. I can't fix it. Maybe you can help me.