Better looking graphs and legend

* Units (SI units atm) on Y axis and legend
* TODO: rework this to change the function used depending of the metric and avoid the ugly hack for rounding
* Display the legend at the bottom of the graph, with min/max/average values
* Click on a metric in the legend hides/displays the metric in the graph
Merge remote-tracking branch 'upstream/master' into better_graphs

Conflicts:
	app/assets/javascripts/app.js.coffee
	app/assets/stylesheets/app.css.less
Woops, fixed stylesheets
Forgotten a fontawesome variable
Fix the order of metrics when hidding/showing one

Also, you can no longed hide the last remaining metric
Allow to toggle the legend for a graph
Merge pull request #28 from renchap/better_graphs

Better looking graphs and legend