Virtual host should be part of URI

As per http://www.rabbitmq.com/uri-spec.html, virtual host parameter is a part of URI.
There is no need to have a separate argument to GELFRabbitHandler, when it can be passed
as a part of URI.

Updated rabbitmq.py and README.rst to include examples
Fixing test for empty string
Add virtual_host argument back, to keep backward compatibility

If the virtual host is specified in URI, it will always take precedence
Merge pull request #39 from dmuntean/master

Virtual host should be part of URI