add CSS4 selector tests
@document test per https://developer.mozilla.org/en-US/docs/CSS/@document
parser fixes
apply @nessy patch and add test case
update expected results (this test currently fails)
add test case for multi-extends with nesting (another failing test)
upcase references to Exception class
fixing @extend combineSelectorSingle()
increase precision from 3 to 5 (per sass 3.2)
Merge pull request #51 from robocoder/precision-patch

increase precision from 3 to 5 (per sass 3.2)
null handling
Add (failing) unit test for: ``` three: $list; ``` scss 3.1.15 generates: ``` three: null; ``` scss 3.2.x drops the property entirely.
fixes
change list parsing precedence
make null a first class value, fixes #33 #45
Merge pull request #46 from robocoder/css4

CSS4 selector support
Merge pull request #47 from robocoder/multi-extend

@extend fixes