Long Metric Value Type support

Add support for Long Value type for a metric column.

API Changes in PR - 
https://github.com/druid-io/druid-api/pull/17
I think column compression will likely take care of reducing the size. I think it's less confusing to store longSum / count aggregations as long. We probably don't need to provide backwards compatibility here, as long as we can still run longSum aggregations on columns that are floats.