Fix nt2 installation

* avoid deletion of inexistant folder
Merge branch 'numpy_simplified' of https://github.com/joker-eph/pythran into joker-eph-numpy_simplified

Conflicts:
	setup.py
Fix include dir (pythran install directory)
Include <numeric> required for std::accumulate
Fix compilation with clang

(ISO C++03 14.2/4)
PEP8 + cleaning