SL add rspec matcher

[synion]

The reason to add new matcher for RSpec was that I assumed that we can check a few assertions using only one matcher.
Also returned fails was hard to read.

Example of use:

mail = MandrillMailer.deliveries.last

expect(mail).to be_mandrill_email(
  template_name: 'user-registered-for-event',
  to: 'user@mail.com'
)

 expect(MandrillMailer.deliveries).to include(
   mandrill_email(
     template_name: 'user-registered-for-event',
     to: 'first_user@test.com'
   ),
   mandrill_email(
     template_name: 'send-invitation-to-user',
     to: 'joe@doe.com'
   ),
   mandrill_email(
     template_name: 'send-invitation-to-user',
     to: 'rejected@user.com'
   ),
   mandrill_email(
     template_name: 'send-invitation-and-paid',
     to: 'simon@templar.com'
   ),
 )
expect(mailer).to be_mandrill_email(
 template_name: 'user-registered-for-event',
 to: 'joe@doe.example',
 global_variables: {
   'FNAME' => 'John',
   'START_TIME' => '10:45am EST / 7:45am PST',
   'VENUE' => 'Midtown Venue',
   'START_DATE' => 'Wednesday, 01/10/2018',
   'NEXT_EVENT_DATE' => nil,
   'MEMBER_FULL_NAME' => 'John Gates',
   'MEMBER_EMAIL' => 'joe@doe.example'
 }
)

Returning info could look like that:

[wwahammy]

This is awesome @synion!