Adding basic metrics support

[rhart]

Initial commit for your review on the approach. I chose to use AOP as it means less noise in the code and will be more flexible going forward than using Guice method interceptors e.g. for non Guice objects. Can easily be changed though if the extra dependencies and/or JVM arg required to activate are a problem.

Usage would be as follows

ratpack {
    modules {
        register new MetricsModule()
        register new MetricsJmxReporterModule()
    }

    handlers { MetricRegistry metrics ->
        get {
          render groovyTemplate("index.html", title: "My Ratpack App")
        }

        prefix("foo") {
            get {
              metrics.meter("foo get handler").mark()
              response.send("foo")
            }
        }

        prefix("bar") {
            get {
                render groovyTemplate("index.html", title: "My Ratpack App")
            }
        }

        assets "public"
    }
}

You can see that custom metrics would be easily supported. Would also intend on providing an annotation to make it even easier.
Additional modules would be created to support the other reporters.

JVM arg is required though to active AOP

run {
    jvmArgs "-javaagent:${configurations.runtime.files.find {it.name == 'aspectjweaver-1.7.3.jar'}.path}"
    systemProperty "ratpack.reloadable", "true"
}

Feel free to tear into it and I will make any changes :)

[rhart]

I've made the changes from your comments but as usual this pull request is looking messy now. Do you want me to create a new one? Not sure what I'm doing wrong but will try and find out.

The main commit of interest is this one rhart@afd9c0e

It's really hard to hook into the close event stuff from the aspect. I just don't have access to the Context. So, I've created a new statefull handler in the Netty pipeline before the Ratpack one. I think this will give a more accurate response time actually. Hopefully you think this is the way to go too.

[ldaley]

I think the commit history mess might be from rebasing on pull instead of merging.

I'm going to pull this and I'll clean it up on my side.

[ldaley]

Pulled.