Add Google Analytics script on site. Closes #19.

* Add Google Analytics source template.
* Add Google Analytics template in base template.
* Add USE_GOOGLE_ANALYTICS to enable/disable script on environments.
Merge branch 'issue#19-google_analytics' into develop
Change recipe style and models. Closes #20.

* Add migration, admin, model and fixtures configuration for Step and Material models.
* Change recipe style and mixture detail too.
Merge branch 'issue#20-recipe_style' into develop
Merge branch 'develop'