fix segfault and memory leak in OctreePointCloudSearch

This is the segfault reported in #28.

__cinit__ unconditionally calls its base class version, so two objects
got allocated and one of them was leaked. Fixed by moving the actual
allocation to __init__.

Similarly, __dealloc__ would call its base class version and a "double
delete" would follow. Fixed by removing the child class's __dealloc__,
and set self.me to NULL explicitly in OctreePointCloud.__dealloc__ for
added safety.