Enhance node status feature in overview and nodes

This commit uses the new parameter with_status from nedap/pypuppetdb#18

Node status is now shown as text with the additional information of
failed/succeded events, unreported time

The statistics on Overview now show the *number of nodes*
that have status failed/changed/unreported
nodes view filter now works with new status attribute
change sorting of the overview node status in javascript
Merge pull request #28 from digitalmediacenter/improve-nodestatus

Enhance node status feature in overview and nodes
overview: Add a count, info if nothing is changing

The Nodes status detail now shows for how many nodes we have events.
Additionally when there are no events we simply show an alert that
nothing is going on.
app: Abort if we can't fetch metrics.