Add editing user spec.
Fix edit form.
add subject line and body specs for Spree::UserMailer

Fixes #106
Add default_url_options to user_mailer spec
Force Spree.user_class to be set to Spree::User to fix the infamous error:

rake aborted!
Association named 'spree_roles' was not found; perhaps you misspelled it?

It's tripped me up WAY too many times now.