Add test, CHANGELOG and docs for auto-cpu detection PR

CHANGELOG

@@ -2,6 +2,8 @@
 -------------------------
 
 - release as wheel
+- "-n" option now can be set to "auto" for automatic detection of number
+  of cpus in the host system. Thanks Suloev Dmitry for the PR.
 
 1.12
 -------------------------

README.txt

@@ -59,7 +59,8 @@ To send tests to multiple CPUs, type::
     py.test -n NUM
 
 Especially for longer running tests or tests requiring
-a lot of IO this can lead to considerable speed ups.
+a lot of IO this can lead to considerable speed ups. This option can
+also be set to ``auto`` for automatic detection of the number of CPUs.
 
 
 Running tests in a Python subprocess

testing/acceptance_test.py

@@ -363,6 +363,19 @@ def test_terminate_on_hangingnode(testdir):
     ])
 
 
+def test_auto_detect_cpus(testdir, monkeypatch):
+    import multiprocessing
+    monkeypatch.setattr(multiprocessing, 'cpu_count', lambda: 3)
+    testdir.makeconftest("""
+        def pytest_unconfigure(config):
+            with open('cpus', 'w') as f:
+                f.write('cpus = %s' % config.option.numprocesses)
+    """)
+    testdir.inline_run('-n=auto')
+    cpus_file = testdir.tmpdir.join('cpus')
+    assert cpus_file.read() == 'cpus = 3'
+
+
 @pytest.mark.xfail(reason="works if run outside test suite", run=False)
 def test_session_hooks(testdir):
     testdir.makeconftest("""