allow to customize the key on a range aggregation

The range aggregation allow to customize the key for each range :

The documentation about this could be found here :
http://www.elasticsearch.org/guide/en/elasticsearch/reference/master/search-aggregations-bucket-range-aggregation.html#_keyed_responsea

Now, the Range aggregation class has a thrird (optionnal) parameter to the `addRange` method
to passe the range customized key name.
Merge pull request #728 from agallou/range_key

allow to customize the key on a range aggregation
Flattr Button added to README