Fix conversion of null.

Closes #709
Merge branch 'stable'
Make null.to_sass take an options hash.
Merge remote-tracking branch 'origin/stable' into stable
Merge branch 'stable'
Bump VERSION to 3.2.8.
Merge branch 'stable'

Conflicts:
	VERSION
	doc-src/SASS_CHANGELOG.md
Merge remote-tracking branch 'origin/master'