Updated Number Operations for Unit Math

See [multiplication error](https://github.com/nex3/sass/issues/83). This comes up all the time and the current documentation implies that Sass will do unit conversion for them for everything.
Merge pull request #1261 from Snugug/stable

Updated Number Operations to include Unit Math
Merge remote-tracking branch 'origin/stable' into stable
Bump VERSION to 3.3.8.
Merge branch 'stable'

Conflicts:
	VERSION
	doc-src/SASS_CHANGELOG.md
Fully parse selectors nested within pseudoclass selectors.
Add some failing tests for selector pseudoclass extension.
Add superselector support for selector pseudoclasses.

This also adds a new superselector test file.
Initial support for extending into selector pseudoclasses.

There are still some specificity and unification kinks to be ironed
out.
Track specificity better when extending.
Support specificity ranges for selectors like :matches().
Support simple cases of nested selector pseudoclasses.