Some minor rearrangements.

- Experimenting with breaking the header out of the Block class.

- Split some expression evaluation methods out of the compiler class, as
  none of them actually depend on anything but the library or each
  other.

- Renamed some poorly-named variables.
Remove spawn_rule.
Remove backwards-compatibility from SassRule.
Add a simple smoketest, which actually covers imports.
Break file handling out of rules and into a tiny class.
Remove SassRule.path.