[WIP] ENH: Parallelize neighbors search through multiprocessing

I used the most simple approach with `'multiprocessing'` joblib backend. A simple benchmark could be seen [here](http://nbviewer.ipython.org/gist/nmayorov/7531d9b59608ae2d76dc).

This is the issue #3536.
It was annotated in code as `# Numpy 1.3-1.4 compatibility utilities`, on the main page of scikit-learn project we see `The required dependencies to build the software are NumPy >= 1.6.2, SciPy >= 0.9 and a working C/C++ compiler.` So I figured it's fine to remove it. 

BTW, I removed all `get_memview_DTYPE` functions.