Check for last page

The `next` key does not disappear from the last page's response so check if it's `null`.
Base undefined limit on response

If the limit option hasn't been set use the count returned by DRF to set
this variable.
Merge pull request #2 from ghickman/patch-1

Check for last page
Merge pull request #3 from ghickman/default-limit

Base undefined limit on response