New AuthTokenFile class for managing the auth_token file
use a different ENV var for testing
use AuthTokenFile for filename
refactor specs
add AuthTokenFile#read
refactor specs
update gist to use AuthTokenFile#read
add AuthTokenFile#write
update gist to use AuthTokenFile#write
AuthTokenFile needn't be a class (no state). just functions
no require-relative
1.8-compatible IO api
Merge branch 'master' into auth_token_filename

* master:
  1.8 hash syntax
  no require_relative
move AuthTokenFile into gist.rb
Merge pull request #186 from jasonkarns/auth_token_filename

extract AuthTokenFile domain object