Token printing

This set of changes add printing of tokens

A token is a label with a QR code, Nickname, Specialty and Superpower

GET people/1.pdf will return the token image as a PDF that can be printed on a label printer
GET people/1/print_token will print the label on the default printer connected to the server

The media to print on is select in config/initilizers/qr_code.rb
Rails.config.token_media
Current supported values:
DC03 => 29 X 90 mm address labels
DC04 => 38 X 90 mm large address labels
A4 does not currently work.