Extract core extensions into a Utils module

Fixes #2111
Add Utils.stringify_hash_keys
Rename those Utils functions.
Use jekyll-sass-converter 1.0.0.rc3
Merge pull request #2112 from jekyll/no-core-ext
Update history to reflect merge of #2112 [ci skip]
Use the rake helper for mkdir as this version makes two folders on Windows
Merge pull request #2118 from mscharley/windows-support
Update history to reflect merge of #2118 [ci skip]