Fixes issue with missing prev_page link

This addresses #JOINDIN-379

On Pages with only one result the link to a previous page was missing
Adds total count of events

Addresses #JOINDIN-397.

This commit introduces a second SQL-Query that counts all matching
events. That total count is then added to the returned value and also
used to check whether there is a next page to be displayed or not
based on the requested LIMIT-parameters.

This changes the returned value in two ways:

* A new parameter ```meta.total``` is added to the returned values
* On certain occasions the next-link is not present where it was present
  before.
Adds some refactoring

Addresses #JOINDIN-397
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'.
Adds total-count to ALL mappers

Addresses #JOINDIN-397

This Commit adds a total-count to all mappers available
removing the first and last results

Not sure we need these additional fields and they are also at odds with
the counts in the returned pagination links
Merge branch 'heiglandreas-feature/JOINDIN-397'