Refactor python/tidy.py to not use globals. Fixes #3242
Cleaned up python/licenseck.py

- Moved logic into tidy.py
- Removed explicit BSD license exceptions, they are already ignored by
  being inside the codegen directory.
Use os.path.join instead of manually inserting slashes.
Remove needless rust-png dependency.
Use virtualenv2 in tests/wpt/run.sh if installed
Add lint for type-checking transmutes
Fix layout of Create Account / Login when rendering Wikipedia.

When calculating the preferred width for a block, accumulate
the left and right float widths of children separately, which
is then max'ed with the normal flow widths later on.

Ref bug #2554 - improves the layout of the top bar.
Merge pull request #3263 from duncankl/refactor-test-tidy

Refactor python/tidy.py (updated post-cargoify)
Merge pull request #3272 from glennw/wikipedia-layout

Fix layout of Create Account / Login when rendering Wikipedia.
Merge pull request #3258 from Manishearth/transmute_lint

Add lint for type-checking transmutes; r=jack
Merge pull request #3268 from saneyuki/dep

Remove needless rust-png dependency.
Merge pull request #3271 from duncankl/virtualenv2-fix

Use virtualenv2 in tests/wpt/run.sh if installed