Add a deaths.json data file and corresponding class.

This has just a few death records to start with, more will be added later.
Add routes to access API for death records.
Add tests for deaths.json data file.
Changes to death records.

- Add "suicide" as a possible cause.
- Merge "assassination" into "murder".
- Permit multiple killers per death record, i.e. where it isn't clear which individual struck the killing blow.
Add ability to get a list of people killed by a given person.
Remove puts statements.
Add more death records.
Add more death records.
Return death and appearance info with each person record.
Remove console.log statement.