Frontend styleguide, adding index

Change-Id: I92cc0728822a9c778ae4e6e6a3d4805f7736117e
Importing SASS/Compass environment for development

* Use of rbenv for specific Ruby version (optionnal)
* Use of Gemfile for specific Compass/SASS version and extensions
* config/compass.rb flip config switches depending of `-e production` environment
* Changes made in sass/*.scss goes into src/documents/assets/css

Things to know
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* Use of rbenv is optionnal, file .rbenv is there in case of need
* Should work with GUI Compass/SASS compiler such as ScoutApp (TO TEST)
* Changing a file in sass/*.scss generates a CSS file in src/documents/assets/css

To improve
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* After change in src/documents/assets/css doesn’t get trigerred by `docpad run`
* Compilation with sass doesn’t minify —making a one-line mess, perfect for production— like gulp does