Add support for Attach API for JMX metrics

This request adds support to enable JMX via the Attach API (http://docs.oracle.com/javase/7/docs/technotes/guides/management/agent.html).

The benefit of this approach is that in order to monitor local JVM processes users won't need to enable the remote JMX monitoring explicitly nor restarting their applications to make changes.

The agent will connect to a local process through the process_name_regex property in the jmx.yaml file and check if JMX is already running in that VM. If JMX is not running the agent will attempt to dynamically load JMX through the Attach API and start getting metrics immediately. 

This is the same way as other official Java tools such as Jconsole and JvisualVM connect to local process and gather data. In order to check against which value the regex will be evaluated, you need to run `jps` command  on a machine with Java processes running.

To wrap up I'd suggest to do some major rework in jmxfetch testing . As a collaborator It's very difficult to introduce new changes without knowing if you're breaking something. I see that many of your integrations (such as Tomcat, ActiveMQ, Java, Cassandra, Solor, etc) rely on this jmxfetch module so I strongly recommend you to put some effort on improving it.