Fix mail preview for cookbook follower notification

The mail preview method was calling the wrong mailer method and
we also need some seed data for this preview to work.
Add text mailer layout

Add text mailer layout with the common footer.
Absolute path for cookbook version notification

Mailers require absolute paths so this links to the full path of the
cookbook version notification.
Add missing cookbook version notification text format

Add text format of cookbook version notification email.
Merge pull request #280 from opscode/mailer_fixes

Mailer fixes