Investigate and choose a CSS framework

[Mathieu Agopian]

Following our decision to not support theming (see #98 and rationale), we also decided to use a CSS framework to have better styling.

Some options:

• bootstrap
• material-ui (see #39)
• zurb foundation

Once a solution has been chosen, use it, and document the fact that we're using it (and that it's thus very simple to re-use themes/css/js to change the look and feel, layout, behaviour...).

[Nicolas Perriault]

Bootsrtap has been selected, and docs were updated.