Define a default mapping for RSS documents

We want to define a default mapping for RSS documents if no one is already provided.

Mapping looks like:

```javascript
{
  "page" : {
    "properties" : {
      "feedname" : {
        "type" : "string",
        "index" : "not_analyzed"
      },
      "title" : {
        "type" : "string"
      },
      "author" : {
        "type" : "string"
      },
      "description" : {
        "type" : "string"
      },
      "link" : {
        "type" : "string",
        "index" : "no"
      },
      "source" : {
        "type" : "string"
      },
      "publishedDate" : {
        "type" : "date",
        "format" : "dateOptionalTime",
        "store" : "yes"
      },
      "location" : {
        "type" : "geo_point"
      }
    }
  }
}
```

Closes #20.