New more configurable version of JMX exporter, using simpleclient.

It supports:
- Scraping the local jvm
- Scraping a remote jvm
- The existing output format by default (mostly, escaping is a bit safer, and it doesn't assume 'type' is the first beanProperty)
- Customising the name, help and labels as needed
- Exporting only what you want (only whitelist right now, blacklist would be easy to add)
- The java simpleclient

A config of 
  { "hostpost": "localhost:1234",
    "rules": [
      {"^java"}
    ]}
would export everything under java, with the default (i.e. existing) format.



I haven't written tests or the servlet bit yet, but wanted to get initial feedback on the approach.
It also puts out a metric per sample too rather than combining metrics with the same name, which isn't correct.
Hmmm, so far, as far as I have seen, in Prometheus lingo, an "exporter" is a stand-alone binary that exports something as Prometheus metrics, while a "collector" is an interface within a client library to collect metrics. I would like to avoid mixing those two up. `node_exporter` exports node attributes, `haproxy_exporter` exports haproxy metrics, so `jmx_exporter` exporting JMX metrics as Prometheus metrics doesn't sound terribly off...