Fix USAGE file indentation
Fix a typo in documentation's README wan -> want
Move to a OOScss-like architecture

Move the whole sass folder to a OOScss-like architecture. The previous
folder's organization was a one level deep and a bit messy. The folder
is now composed of the following folders:

  - components : UI elements (e.g. alerts, buttons)
  - layout : stuff related to the layout basics (e.g. grid, header)
  - responsive : stylesheets related to specific devices
  - settings : variables, mixins and reset partials
Move to semantic versioning and bump to 2.0.0

The next version will be 2.0.0. A lot of things is going to change
so let's ship with 2.0.0 as the next release.
Add release date for previous versions
Move mixins related to components to placeholders

Move mixins such as `label` or `alert` into their respective file and
change them to a placeholder. The mixins should only be used to avoid
code duplication for properties (e.g. prefixed ones) and not for the
styling of a component.
Make header's button selector a bit more flexible

Previously, the list of available buttons was hard-coded directly in
the stylesheet file ; it made new buttons (implemented by the user for
example) not stylized correctly in headers.
Add a contribution guideline