Implement flavor helper to build multiple versions of the source.

Set the FLAVOR environment variable to target specific flavors.
For example, to build the source for Alve flavor run:
  `FLAVOR=alve middleman build`
The default flavor is 'skroutz'.
Rename main stylesheet to de-flavor it
Add style flavors for the default layout
Add style flavors for index page
Make index page multi-flavored
Make footer and its partials multi-flavored
Multi-flavored Authentication pages
Multi-flavored Analytics pages
Translate to English API v2 Overview page
Translate to English the API v2 Data Model page
Translate in English API v2 Product page
Translate in English API v2 Books page
Translate in English API v2 User page
Translate in English API v2 Widgets page
Multi-flavor Help
De-flavor Guides page
Multi-flavored Guides Getting Started page
Multi-flavored Authentication page
De-flavor Permissions page
Translate in English OAuth Overview page