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.
exchanges recreated if vhost is not default /

This should fix issue 173:
https://github.com/puppetlabs/puppetlabs-rabbitmq/issues/173

self.instances would only return the list of exchanges for the default
vhost /, this would cause a problem as any other exchange would be
recreated in every Puppet run.
Also, rabbitmqctl was used instead of rabbitmqadmin because the latest
requires the username and password of the admin user to list the
exchanges, while the first does not.