JENKINS-23933: Expose build successful/unstable/failed metrics.

We are running a couple of Jenkins masters. We have run into problems because nodes are not able to access vital systems like our internal Maven repository or our SCM, sometimes because the hosts were down or because of new VLAN settings or firewalls between nodes or even because of configuration changes.
In these cases the number of failing builds is increasing immediately. It would be nice if the metrics plugin could provide metrics of successful, unstable and failed builds, so we can react more promptly.
Merge pull request #3 from mfriedenhagen/JENKINS-23933

JENKINS-23933: Expose build successful/unstable/failed metrics.