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.
Get rid of the now-redundant Literal#{space,comma} methods.
Merge branch 'stable'
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
Add a list-separator function.

Closes #689
Add a changelog entry for list-separator.
Merge branch 'stable'

Conflicts:
	lib/sass/exec.rb
	lib/sass/script/parser.rb
Always return nil in case of errors in cache store/retrieve.

Closes #705
Merge branch 'stable'