Support for rootPrefix in yaml2jmxtrans.py

I'm not a Python developer, and this project is my first fork on GitHub. 
Although I don't think I've broken anything, please to take a careful look at this commit.

I needed a way to change the default "servers" prefix for the Graphite OutputWriter. OutputWriter already had a rootPrefix attribute, but yaml2jmxtrans.py appeared not to support this. This commit adds support for rootPrefix.

yaml2jmxtrans.py allows one to specify multiple JMX host:port;alias combinations per set.
I've adapted this to allow the alias to be optionally delimited with a forward-slash (/).
The second part of the alias is the metric's rootPrefix.

```
sets:
  - setname: set1
    query_names:
            - threads
    hosts:
            - tomcathost-dev:58080;tomcathost-dev-8080/stats.jvm.dev
            - tomcathost-uat:58080;tomcathost-uat-8080/stats.jvm.uat
```

Using the delimited alias is optional, and omitting it causes the current default prefix, "servers", to be used.