Update README.md

Add documentation for exchanges native type
Merge pull request #179 from jaimegago/master

Update README.md
Fix unit tests.

Apt has changed and now needs an lsbdistid passing through.
Merge pull request #182 from apenney/fix-unit-tests

Fix unit tests.
allow port to be 'UNSET'.

This allows to have a setup where rabbitmq is not listening
on an non ssl port.
eg:
  class { '::rabbitmq':
    ssl                      => true,
    ssl_cacert               => '/etc/rabbitmq/ssl/cacert.pem',
    ssl_cert                 => '/etc/rabbitmq/ssl/cert.pem',
    ssl_key                  => '/etc/rabbitmq/ssl/key.pem',
    config_variables         => { tcp_listeners => '[]' },
    port                     => 'UNSET',
  }