implement and document new invocation mechanisms, see doc/usage.txt

also rename pytest._core to pytest.main for convenience.

setup.py

@@ -46,7 +46,7 @@ def main():
     )
 
 def cmdline_entrypoints(versioninfo, platform, basename):
-    target = 'pytest:__main__'
+    target = 'pytest:main'
     if platform.startswith('java'):
         points = {'py.test-jython': target}
     else:
@@ -66,4 +66,4 @@ def make_entry_points():
     return {'console_scripts': l}
 
 if __name__ == '__main__':
-    main()
+    main()