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.
prometheus client library handles NaN ok?