Encourage "python -m tornado.test.runtests" as the way to run the tests.

Remove execute bit from tornado/test/runtests.py, which would unexpectedly
run the tests from a globally-installed version of tornado when run directly.
Add a new runtests.sh to make the recommended behavior more discoverable.
Remove run_pyversion_tests.py, which has been superceded by tox.
Update docs.

Closes tornadoweb#545.

MANIFEST.in

@@ -9,4 +9,5 @@ include tornado/test/gettext_translations/fr_FR/LC_MESSAGES/tornado_test.mo
 include tornado/test/gettext_translations/fr_FR/LC_MESSAGES/tornado_test.po
 include tornado/test/static/robots.txt
 include tornado/test/templates/utf8.html
+include runtests.sh
 global-exclude _auto2to3*

runtests.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+cd $(dirname $0)
+
+# "python -m" differs from "python tornado/test/runtests.py" in how it sets
+# up the default python path.  "python -m" uses the current directory,
+# while "python file.py" uses the directory containing "file.py" (which is
+# not what you want if file.py appears within a package you want to import
+# from)
+python -m tornado.test.runtests "$@"

tornado/testing.py

@@ -388,9 +388,9 @@ def main():
     be overridden by naming a single test on the command line::
 
         # Runs all tests
-        tornado/test/runtests.py
+        python -m tornado.test.runtests
         # Runs one test
-        tornado/test/runtests.py tornado.test.stack_context_test
+        python -m tornado.test.runtests tornado.test.stack_context_test
 
     """
     from tornado.options import define, options, parse_command_line