Limit results from `razor events`

The list of events in `razor events` can grow unwieldy very quickly. This
change instates a default of all rows returned with options to return
different rows using parameters /api/collections/events:
- `limit` determines the maximum number of rows to return,
which defaults to 20.
- `start` determines what row number to start with, which defaults to 0.

Fixes https://tickets.puppetlabs.com/browse/RAZOR-382