Adding OpenTSDB support

Hi,
I've created a writer for OpenTSDB. 
Example configurations:

    {
        "@class": "com.googlecode.jmxtrans.model.output.OpenTSDBWriter",
        "settings": {
            "host": "192.168.250.151",
            "port": 4242,
            "typeNames": ["name"],
            "tagname": "type"
        }
    }

When using the "typeNames" parameter, the values corresponding to the names will be appended as tags (see tags: http://opentsdb.net/metrics.html).
When you query composite data types, the member values will also be added as tags, where the tag name will be defined by the "tagName" settings, and the tag value will be the key's name of the data member. 
The host name would be automatically appended as a tag.
Could somebody review the patch?