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
drop database tables timestamp fields
Forgotten a fontawesome variable
Merge pull request #29 from josqu4red/remove_ts

drop database tables timestamp fields
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
fix flush of rrd files in collectd provider
Merge pull request #30 from josqu4red/fix_flush

fix flush of rrd files in collectd provider
add timescale support for graphs, add minimal layout
Merge pull request #31 from josqu4red/graphs

add timescale support for graphs, add minimal layout
add dashboards

  - create dashboards & dashboards_graphs tables
  - unify graphs/dashboards display, using dedicated controllers
  - put sidebar in partial to display it from any controller
add system dashboard
add more dashboards
fix dashboard load & remove cleanup task
Merge pull request #32 from josqu4red/dashboards

Dashboards