Add java documentation for aggregations

Buckets:
--------

* terms
* range
* global
* filter
* filters
* missing
* nested
* reverse nested
* children
* significant terms
* date range
* ip range
* range
* histogram
* date histogram
* geo distance
* geo hash grid

Metrics:
--------

* min
* max
* sum
* avg
* stats
* extended stats
* value count
* percentiles
* percentile rank
* cardinality
* geo bounds
* top hits
* scripted metric
Well. You're right and wrong. :) Basically if you are using elasticsearch client (node or transport), execution will happen on the cluster side where data live actually so this dependency is not required.
If you embed a data node in your application (which could be one of the architecture design), then it requires that you add this.
Tests I ran (JUnit with embedded node) failed without this dependency.

So I'm going to add an explanation like "if your are running embedded data node for unit tests for example".