Less use optimisation

Improvement:
* LESS file watcher (using the command `grunt dev`, this command also start the web server and open page preview)
* Add LiveReload support (using the command `grunt watch:css` or include in the command `grunt dev`)
* Add deployement tool (using the command `grunt deploy`, this will be useful once we have a hosting server...)
* Add production version preview (using the command `grunt prod`)

To test this PR, see the README.md file to configure correctly the project.