Fix some division, color and null issues already covered by failing tests

Address a few failing tests by tackling three separate issues; firstly, evaluation of divisions within parentheses (see https://github.com/rowanbeentje/libsass/commit/18181169745cebaeb1caa964a3ce6292e9d35010); secondly, formatting of unmodified colors (see https://github.com/rowanbeentje/libsass/commit/c6e4b86094c12ad54b2b96367bc5f23bafa6d94b); and finally a number of improvements to Null handling, including list output tweaks (see https://github.com/rowanbeentje/libsass/commit/030d0784d93de70ce97d3aaf261f0bd95b1924ed) - I think this builds on the work done by @Nabellaleen in https://github.com/hcatlin/libsass/pull/168.

Note that the color handling code takes an approach of favouring the originally entered values, overriding this in a few places - this is so that when https://github.com/nex3/sass/issues/363 is actioned, the overrides can be easily removed.