Add support for content:encoded field

Some RSS feeds provide the full content of the item in a "content:encoded" field. It would be great if this field could be stored and indexed if provided.

The specific feed I am trying to consume is: https://blog.mariadb.org/feed/
Another feed with content:encoded: http://www.dcrainmaker.com/feed
Description of content:encoded : http://www.rssboard.org/rss-profile#namespace-elements-content-encoded

We add it under `raw.html` stored and not analyzed field.

Closes #25.
Replace update_rate field from long to TimeValue

We want to support `TimeValue` instead of long when it comes to defining the refresh rate, so users will be able to define this value without having to compute to milliseconds.

Was:

```sh
$ curl -XPUT 'localhost:9200/_river/lemonde/_meta' -d '{
  "type": "rss",
  "rss": {
    "feeds" : [ {
        "url": "http://www.lemonde.fr/rss/une.xml",
        "update_rate": 900000
        }
    ]
  }
}'
```

is now

```sh
$ curl -XPUT 'localhost:9200/_river/lemonde/_meta' -d '{
  "type": "rss",
  "rss": {
    "feeds" : [ {
        "url": "http://www.lemonde.fr/rss/une.xml",
        "update_rate": "15m"
        }
    ]
  }
}'
```

Closes #31.