Add a tox config for the py33 backport of asyncio.

Update some old comments.

tox.ini

@@ -1,17 +1,16 @@
-# Tox (http://codespeak.net/~hpk/tox/) is a tool for running tests
+# Tox (http://tox.readthedocs.org) is a tool for running tests
 # in multiple virtualenvs.  This configuration file will run the tornado
 # test suite on all supported python versions.  To use it, "pip install tox"
 # and then run "tox" from this directory.
 #
-# See also tornado/test/run_pyversion_tests.py, which is faster but
-# less thorough.
-#
-# On my macports-based setup, the environment variable
-# ARCHFLAGS='-arch x86_64' must be set when building pycurl.
+# When building pycurl on my macports-based setup, I need to either set the
+# environment variable ARCHFLAGS='-arch x86_64' or use
+# 'port install curl +universal' to get both 32- and 64-bit versions of
+# libcurl.
 [tox]
 # "-full" variants include optional dependencies, to ensure
 # that things work both in a bare install and with all the extras.
-envlist = py27-full, py27-curl, py32-full, pypy, py26, py26-full, py27, py32, py32-utf8, py33, py27-opt, py32-opt, pypy-full, py27-select, py27-monotonic, py33-monotonic, py27-twisted, py27-threadedresolver, py27-twistedresolver, py27-twistedlayered, py27-caresresolver, py32-caresresolver, py27-locale, py27-docs
+envlist = py27-full, py27-curl, py32-full, pypy, py26, py26-full, py27, py32, py32-utf8, py33, py27-opt, py32-opt, pypy-full, py27-select, py27-monotonic, py33-monotonic, py33-asyncio, py27-twisted, py27-threadedresolver, py27-twistedresolver, py27-twistedlayered, py27-caresresolver, py32-caresresolver, py27-locale, py27-docs
 [testenv]
 commands = python -m tornado.test.runtests {posargs:}
 
@@ -179,6 +178,11 @@ basepython = python3.3
 basepython = python3.3
 commands = python -m tornado.test.runtests --ioloop_time_monotonic {posargs:}
 
+[testenv:py33-asyncio]
+basepython = python3.3
+commands = python -m tornado.test.runtests --ioloop=tornado.platform.asyncio.AsyncIOLoop {posargs:}
+deps = asyncio
+
 [testenv:py32-caresresolver]
 basepython = python3.2
 deps =