Add psutil extra and introduce pytest_xdist_auto_num_workers hook

This makes using psutil optional and opens up the possibility of customization
through the pytest_xdist_auto_num_workers hook, making things like #477 possible.

Fix #585

setup.py

@@ -18,7 +18,7 @@ setup(
     platforms=["linux", "osx", "win32"],
     packages=find_packages(where="src"),
     package_dir={"": "src"},
-    extras_require={"testing": ["filelock"]},
+    extras_require={"testing": ["filelock"], "psutil": ["psutil>=3.0"]},
     entry_points={
         "pytest11": ["xdist = xdist.plugin", "xdist.looponfail = xdist.looponfail"]
     },