MAINT: Add a 'count_nonzero' function to use with numpy 1.5.1.
MAINT: linalg: use count_nonzero from lib._numpy_compat
MAINT: optimize: use count_nonzero from lib._numpy_compat in _linprog.py
MAINT: sparse.linalg: use count_nonzero from lib._numpy_compat
Merge pull request #3909 from WarrenWeckesser/count_nonzero

MAINT: Create a common version of `count_nonzero` for compatibility with numpy 1.5.