Fix minimum-specificity calculation when trimming @extend output.

Closes #511.
Bump VERSION to 3.2.7.
Update SASS_CHANGELOG.md: missing space

A little typo.
Merge remote-tracking branch 'origin/stable' into stable

Conflicts:
	doc-src/SASS_CHANGELOG.md
Refer to the correct ivar in the SCSS parser.

I'm pretty sure #rethrow is only called within a #throw_error block, but it
should still behave correctly if it's not.
Add zip and index functions to Sass::Script::Functions Overview, edit function descriptions for clarity.

See issue #676
Merge pull request #696 from robwierzbowski/zip-index-docs

Added zip and index functions to Sass::Script::Functions Overview
Get rid of the now-redundant Literal#{space,comma} methods.
Fix a list parsing bug.
Fix a list conversion bug.

Closes #699
Fix a bug where multiple spaces in a string literal would sometimes be folded together.

Closes #686
Don't open a file for writing until we have something to write to it.

Closes #704
Always return nil in case of errors in cache store/retrieve.

Closes #705
Merge remote-tracking branch 'origin/stable' into stable
Fix conversion of null.

Closes #709
Make null.to_sass take an options hash.
Fix an invalid mixin-loop report.

Closes #714
Merge remote-tracking branch 'origin/stable' into stable
Bump VERSION to 3.2.8.
Fix a mutability bug.

Closes #722.