Handle unfetch() of records

[cowboyd]

Add the ability to pass in an unfetch option that will be invoked whenever a page is unloaded. This allows you to do thing like perform any cleanup associated with those records such as unload the record from a data store:

let dataset = new Dataset({
  fetch: function(pageOffset, pageSize) {
    return this.store.find('color', {page: pageOffset, perPage: pageSize});
  },
  unfetch: function(records) {
    records.forEach(function(record) {
      record.unload();
    });
  }
});

[flexyford]

Worked on this for #12 but implemented

unfetch: function(pageOffset) {
    ...
  }

I'll edit the code to reflect your implementation below

unfetch: function(records, pageOffset) {
    ...
  }

[flexyford]

Fixed on #12