Fix evaluation of divisions within parentheses

Division syntax is ambiguous, as slashes can be used in property values
to separate numbers.  Normal slashes are therefore not evaluated:
15 / 3 / 5 => 15 / 3 / 5
However, if the value is surrounded by parentheses, it should be
interpreted as a divison:
(15 / 3 / 5) => 1
This commit fixes the identification.
Fix formatting of unmodified colors

Colors which have no operations applied to them are normally not
converted to a standard output.  This commit tracks the original color
format and outputs it if no changes have occurred, matching compass
output.

It does this in a slightly roundabout way, mostly so that when
https://github.com/nex3/sass/issues/363 is done on the sass side, a few
lines can be removed and the original format will be kept much more
widely to match.
Improve Null handling to match tests and compass

Improve handling of Null variables in a number of edge cases including
unary expressions, lists, and variable resolution to match expected
output.  List output is also improved to match expected, and selectors
with only null values are now also not ouput (as expected).
Merge pull request #172 from rowanbeentje/master

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