Add tests/conftest.py for py.test

This does for py.test was tests/nose_plugin does for nose, but it's
nicer because there is no plugin that needs to be registered in setup.py
(or no custom test runner to force the use of the plugin)
Make some test classes subclass unittest.TestCase

so that py.test invokes their setUp methods
tox.ini: Use py.test and don't use setup.py develop
.travis.yml: Use py.test
Add pytest.ini
Create mark_slow that can work with nose or py.test
Allow test skips to work in nose or py.test
tox.ini: Remove nose dep
.travis.yml: Remove nose dep
tests/conftest.py: tox: Remove root dir from sys.path

When using tox, it can accidentally pick up a {pymssql,_mssql}.so file
in the root directory and then get ImportError because of
incompatibility in Python versions. By removing the root directory from
the sys.path, it forces tox to import the library from correct place in
the tox virtualenv.