Adds more metadata

Addresses #JOINDIN-397

This commit adds more metadata:

- A new field ['meta']['first_result'] contains the count of the first
  result being returned
- A new field ['meta']['last_result'] contains the count of the last
  result being returned

Together with the ['meta']['total'] now any API-call has all
informations needed to display something like 'Events 6-10 out of 325'.