Add review email

Added ReviewMailer to send email to user after a review is posted on a truck they added.
Don't create records outside of `it` or `scenario` blocks, these will stick around in your test database. You can use the `let` statement to create these objects as they are needed in the tests:

```ruby
describe Review do
  let(:user) { FactoryGirl.create(:user) }
  let(:review) { FactoryGirl.create(:review) }

  it 'sends an email' do
    expect { review.send_email }.to change { ActionMailer::Base.deliveries.count }.by(1)
  end
end
```