JSON requests for data should respect pagination options

[rhyolight]

When users go to http://localhost:8085/<river-name>/data.json, two new pagination options should apply to the search results:

• itemsPerPage int: how many keys to show on a page
• page [int]: which page to show

If no page query param is received, the default should be 1.

The resulting JSON should also be updated to contain the page in result objects, like this:

{
  "name":"<river-name>",
  "type":"<data-type>",
  "timezone":"<timezone>",
  "page":"1",
  "headers":[...],
  "data":[...]
}

Depends on #129.