Declare namespace to setuptools if it is installed.

I think this should address the issues from #8.

It is all confusing for a few reasons:

- The setup.py from ``backports.ssl_match_hostnamez`` uses setuptools while the setup.py from ``backports.lzma`` uses distutils.
- The README from the [original backports module](https://pypi.python.org/pypi/backports) suggests that you should use a particular template in the ``backports/__init__.py`` file in reference to a PEP... but that pep [was rejected](http://legacy.python.org/dev/peps/pep-0382/).
- That pep suggests that setuptools provides its own roughly equivalent workaround, one which of course only works with setuptools.

So, in 0f36997 I try first to use the setuptools workaround (which seems to be necessary if setuptools is installed?) and if it is absent, then fall back to the phrase put forward in [pep-0382](http://legacy.python.org/dev/peps/pep-0382).