Add tox.ini.
Merge pull request #12 from nvie/patch-1

Always quote attrs ending with a slash.
Merge pull request #13 from nvie/add-tox

Add tox.ini
Use StringIO to build strings more efficiently.

See this article for a performance comparison:
    http://www.skymind.com/~ocrow/python_string/

On the test suite, it does not make much of a performance difference,
but using this on large HTML files should be a little quicker (and take
less memory).  The string concatenation is effectively run at the
C level.