Compute standard deviation of losses

Implements https://bugs.launchpad.net/oq-engine/+bug/1216867

Requires https://github.com/gem/oq-risklib/pull/60/ and https://github.com/gem/oq-nrmllib/pull/104

Summary of changes:

1) Moved the calculation of this kind of loss curve statistics (average loss and standard deviation) to risklib
2) Extended the output_type `loss_curve` to `event_loss_curve` which is a `loss_curve` that includes also standard deviation of losses