Updates schema to use unique name identifier

- Adds an `identifier` attribute to the `bucketeers` table that uniquely refers to a person. Currently this is just a person's name sans spaces, but we could use Facebook or Twitter handle to standardize. This attribute was added so that we could have multiple people named, e.g., 'Tim Cook', and still refer to them uniquely using a human-readable mnemonic.

- Updates bucketeers.yml to include the identifier attribute and corrects Tim Cook's and Phil Schiller's names.
Adds identifier uniqueness constraint
Adds a simple show view

- Adds a show action to BucketeersController that finds people by identifier
- Displays the show route as `bucketeers/{:identifier}`