Gaps marking aggregator

This aggregator will insert a NullDataPoint (that serializes with a null as JSON value) for time ranges corresponding to the sampling with no Data point.
It allows plotting efficiently data gaps according that the expected sampling rate of the metric is known by the user.

The code is not fully tested, but more a proof of concept. If the pull request is merged it would be nice having someone reviewing the code.

Cf. discussion in kairosDB group : https://groups.google.com/forum/#!topic/kairosdb-group/nuOQCF2JdJ0