Add century interval in Date Histogram facet

Heya

I would like to have a century interval option in the Date Histogram Facet.

```javascript
{
    "query" : {
        "match_all" : {}
    },
    "facets" : {
        "histo" : {
            "date_histogram" : {
                "field" : "field_name",
                "interval" : "century"
            }
        }
    }
}
```
Closes #2473.

Cc @uboness: what do you think? Can we add it in 0.90 and master as well? If so, could you add this option as well for the aggregation feature?

BTW JODA does not seem to support decades. That's why I removed this option from the original feature request.