minor feature: histogram panel timezone support

the histogram timezone problem: histogram does not treat timezone correctly. The problem occurs under following condition:

1. local timezone is +0800;
1. elasticsearch index is sharded by utc time;
1. the index pattern is set to: DAY [logstash-]YYYY.MM.DD
1. @timestamp condition is set from 2014-04-13T16:00:00.000Z and 2014-04-14T15:59:59.000Z.
1. I config histogram panel to show total count of data per 1d

now here goes the problem:
histogram panel bar of 2014-04-14 +0800(my local day) showed count data from 2014-04-14T00:00:00.000Z to 2014-04-14T15:59:59.415Z, which is not correct. the right answer should be count data from 2014-04-13T16:00:00.000Z to 2014-04-14T15:59:59.000Z.

the reason of this problem: kibana do treat timezone wisely, but in this situation, kibana is just confused by elasticsearch response. first, kibana sent request localhost/logstash-2014.04.14/_search?search_type=count and elasticsearch sent back:
```javascript
{"took":1,"timed_out":false,"_shards":{"total":5,"successful":5,"failed":0},"hits":{"total":837,"max_score":0.0,"hits":[]},"facets":{"0":{"_type":"date_histogram","entries":[{"time":1397347200000,"count":24},{"time":1397433600000,"count":812}]}}}
```
then kibana sent another request localhost/logstash-2014.04.13/_search?search_type=count and elasticsearch sent back:
```javascript
{"took":1,"timed_out":false,"_shards":{"total":5,"successful":5,"failed":0},"hits":{"total":432,"max_score":0.0,"hits":[]},"facets":{"0":{"_type":"date_histogram","entries":[{"time":1397347200000,"count":24}]}}}
```
finally, histogram panel will compute the total count of 2014-04-14 00:00:00(1397433600000) to be 812, and the 24 will belong to 2014-04-13 00:00:00(1397347200000)

Solution to above problem is straightforward: elasticsearch natively support timezone parameter when build query facet. [related API](http://www.elasticsearch.org/guide/en/elasticsearch/reference/master/search-aggregations-bucket-datehistogram-aggregation.html). Simply add this timezone parameter will fix the problem.