Fixed #13 -- Added support for running tests using tox.

Added tox to enable the developers to run tests against multiple versions of Python (fixes #13).

There's also a `setup.py` integration included, it delivers the consistent experience when tests are being run via `python setup.py test`.

There's a known issue with a `test_too_big_value` test failure, but it is caused by the test incompatibility with pylibmc-1.3.0 (default for `compress_level` had been changed from 1 (`Z_BEST_SPEED`) to -1 (`Z_DEFAULT_COMPRESSION`) in 1.3.0), so it fails even without the changes that are being pull-requested. I plan on opening a separate issue for that.

Thank you.