Change-Id: Ie96edd05e962663e9ddeeda85b24858caabd3bfd
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/262

profiling problems here
- add extras_require to setup.py for the most common DBAPIs
- rework tox.ini to use extras, specify a test matrix built in

By telling wheel that we have extension modules, even though we
have none, wheel will create a Wheel which is platform and
interpreter specific. This will ensure that the pure Python wheels
on PyPy do not trigger installs on CPython without the C speedups.

- clean up other things we aren't using anymore

- add first-class "pytest" runner to setup.py, fixes #3489
- remove references to setuptools Feature and mark that "--without-cextensions"
 is removed, fixes #3500

distutils appropriately, and errors would be emitted at the end
of the test suite.

removal of the ``setuptools.Feature`` extension from setuptools.
If this keyword isn't present, the setup will still succeed
with setuptools rather than falling back to distutils.  C extension
building can be disabled now also by setting the
DISABLE_SQLALCHEMY_CEXT environment variable.  This variable works
whether or not setuptools is even available. fixes #2986
- using platform.python_implementation() in setup.py to detect CPython.
I've tested this function on OSX and linux on Python 2.6 through 3.4,
including 3.1, 3.2, 3.3.
Unfortunately, on OSX + 3.2 only, it seems to segfault.  I've tried
installing 3.2.5 from the python.org .dmg, building it from source,
and also blew away the whole 3.2 directory, something seems to be wrong
with the "platform" module on that platform only, and there's also no
issue on bugs.python.org; however, I'm going with
it anyway. If someone is using 3.2 on OSX they really should be upgrading.