Fix several more socket-related tests under Py3: test__examples, test__os, test__fileobject, test__subprocess_poll, test__server, test__execmodules.

First pass on socket._fileobject for Py3; this went away, but is used heavily in our code base, so the shortest path is probably going to be to emulate it. This fixes test__backdoor.py. others to come.

One test in test_threading.py fails under PyPy 2.6; that test has been removed from upstream (https://bitbucket.org/pypy/pypy/diff/lib-python/2.7/test/test_threading.py?diff2=4ecd94a66a87&at=vendor/stdlib) so don't run it here either. I'm not updating the whole module yet because we run tests under 2.5.0 on Travis.

subprocess.Popen `bufsize` option actually used

Use the TravisCI container infrastructure and enable linters

Previous debugging shows the socket accepting all the data we send it, which is just under 2MB. Maybe travis containers have very large default socket buffers? Up the data size to about 4MB.

Pre-allocate the (biggec) data before we start taking timing measurements, and be sure to close the socket when done.

Add Tox environments for running linters and include one in the default env list. Add a Tox environment for running leak tests and one for doing both, but do not run them by default. Unify the Travis and Tox configurations, and make Travis run both leak and lint tests. Switch the leak test from using gettotalrefcount (only available in debug builds) to using len(gc.get_objects()); while this is not exactly the same thing, it make be close enough, and it's avaialble to every Python developer unlike gettotalrefcount.

typo in event.py comment