Load ActionMailer only if it is also used.

[ream88]

The working version of #65.

---

I have Rails app which will never use ActionMailer, so I excluded ActionMailer at all:

require 'active_record/railtie'
require 'action_controller/railtie'
# require 'action_mailer/railtie'
# require 'active_resource/railtie'
# require 'rails/test_unit/railtie'
require 'sprockets/railtie'

This commit fixes specs failing with uninitialized constant ActionMailer::TestCase::Behavior::TestHelper (NameError).

[blowmage]

I am partial to using this commit from @terrcin's branch:

terrcin@b2d5233

[ream88]

No problem, just a working solution is needed ;)