suppress DeprecationWarnings

I get these warnings when running on my OS X 10.8.5, Python 2.7.2, and also on an Ubuntu Linux running Python 2.7.3:

```
$ python -W all -c 'import pynliner; pynliner.fromString("<style>p{color:red} p{color:blue}</style><p>foo</p>")'
/Library/Python/2.7/site-packages/cssutils-0.9.10-py2.7.egg/cssutils/css/cssstyledeclaration.py:624: DeprecationWarning: Call to deprecated method '_getCSSValue'. Use ``property.propertyValue`` instead.
  property.cssValue = newp.cssValue.cssText
/Library/Python/2.7/site-packages/cssutils-0.9.10-py2.7.egg/cssutils/css/cssstyledeclaration.py:624: DeprecationWarning: Call to deprecated method '_setCSSValue'. Use ``property.propertyValue`` instead.
  property.cssValue = newp.cssValue.cssText
```

I noticed this when running my Django tests, because Django has more strict defaults for `-W` (the `warnings` module).

This fix gets rid of those warnings, but might break compatibility with old versions of eg. cssutils. OTOH, without this fix, something might break with a future version of cssutils.