Support presentational hints

[liZe]

See #253.

This feature has been implemented in a clean way.

Random thoughts:

• I've added this feature because it's useful for many W3C tests.
• The diff is big but really simple.
• Many hints are given in CSS in the spec, they could (should?) be added in another stylesheet in WP with priority set to 0 (instead of code).
• Merging find_style_attributes and find_presetational_hints is possible, it would avoid going through the whole tree twice.
• The code is not tested, but the test suites available on test.weasyprint.org pass without a crash.
• Calling check_style_attribute with constant strings is probably useless.
• This features makes the tests last ~5% more for me, and it's probably much worst on real cases. Should we allow the users to enable/disable it with an option?
• Some rules have not been implemented, mainly because they would be much easier to handle with CSS than with code (one more reason to use an extra stylesheet).
• Some parsing errors are logged, some aren't (who cares?).

Comments are welcome, as usual. I won't merge it right now, I'd like (at least):

• a new option to activate this feature (and keep it deactivated by default),
• a stylesheet for rules described in CSS in the spec,
• tests.

[liZe]

This PR is ready to be merged. I'm happy with it because:

• it's optional and doesn't impact performances by default,
• its code is clean 💐, with CSS rules when possible and Python code when it's not,
• it's tested and quite reliable,
• it's implementing almost all the rules defined in the spec,
• it's really useful for the W3C tests.

The main problems remaining are not that important:

• CSS rules' attributes values are case-sensitive (we have to implement CSS Selectors Level 4 to fix this),
• some values should raise errors instead of being accepted or silently rejected.

@SimonSapin I think that you don't care about presentational hints, but you can look at the code if you want 😄.