support anomaly detection

[int-chaos]

Add a task type 'anomaly_detection', and at least one learner for it.

From my understanding there are multiple types of anomalies - point anomalies, contextual anomalies, and collective anomalies - and also different types of anomaly detection algorithms - nearest-neighbor, clustering, classification, and statistic based.

If anyone has any suggestions or requests for this task, please comment.