README update (small typos)

Signed-off-by: Jean-Denis Vauguet <jd@vauguet.fr>
s/Bower/npm/ + Grunt

Signed-off-by: Jean-Denis Vauguet <jd@vauguet.fr>
gitignore node_modules

Signed-off-by: Jean-Denis Vauguet <jd@vauguet.fr>