TODO

Tons of stuff to do:

- CSS validation.
   - Make sure all generated CSS styles are correct.
   - Make sure all colors, percentages, etc. are correct.

- First class types for lots more things
   - Color type (supports arithmetic)
   - Percentage type (postpend '%', support arithmetic, do range checking)
   - em's, ex's, etc. for other CSS units.
   - margins.  Smart (and correct CSS) behavior for something like this:
       m1 = pyssed.style('margin: 3px')
       m2 = m1 + pyssed.style('margin-right: 0px')

       when used, m2 should output something like "margin: 3 0 3 3"
   - Borders (see support for margins)
   - fonts.  Support the single-line font syntax automatically.

- Better ordering
   - Use OrderedDict or something so that output ordering is predictable and
     matches the way it's specified in Python.

- Better support for pseudo-attributes :hover, :visited, etc.
   - There should be a nice syntax for this where you don't have to repeat
     yourself so much.

- Better error messages
   - Current error messages are really really weak.

- Support for more operators
   - "-" operator __sub__() should work as you might expect it to
   - "&" should be the same as "+"