Only send confirmation email to volunteers

https://www.pivotaltracker.com/story/show/41278297
Rsvp mailer: Refactor volunteer_confirmation

per tjgrathwell's comments in
https://github.com/railsbridge/bridge_troll/pull/143
Add some detail about using MailCatcher for email
Rsvp mailer: render the body

I was using some fancy footwork to send confirmations only to
volunteers, and that interfered with Rails' default mailer template
nomenclature.  Pointed the mailer to the right template, added some
tests to catch this in the future. Added location to the event
factory to enable tests.
Merge branch 'rsvp-confirmation-mailer' of https://github.com/k-stewart/bridge_troll into k-stewart-rsvp-confirmation-mailer