Nodes List JSON API

This PR returns a list of nodes information under http endpoint '/grid/api/nodes'

An example of the JSON output:

    GET: http://localhost:4444/grid/api/nodes

Results:

````
{
    "nodes": {
        "http://10.37.129.2:5555": {
            "id": "http://10.37.129.2:5555",
            "capabilities": [{
                "platform": "MAC",
                "seleniumProtocol": "WebDriver",
                "browserName": "chrome"
            }],
            "configuration": {
                "port": 5555,
                "servlets": [],
                "host": "10.37.129.2",
                "cleanUpCycle": 5000,
                "browserTimeout": 0,
                "hubHost": "localhost",
                "registerCycle": 5000,
                "hub": "http://localhost:4444/grid/register",
                "capabilityMatcher": "org.openqa.grid.internal.utils.DefaultCapabilityMatcher",
                "newSessionWaitTimeout": -1,
                "url": "http://10.37.129.2:5555",
                "remoteHost": "http://10.37.129.2:5555",
                "prioritizer": null,
                "register": true,
                "throwOnCapabilityNotPresent": true,
                "nodePolling": 5000,
                "proxy": "org.openqa.grid.selenium.proxy.DefaultRemoteProxy",
                "browser": "browserName=chrome,",
                "maxSession": 5,
                "role": "node",
                "hubPort": 4444,
                "timeout": 300000
            }
        }
    }
}
````


Anyone can give me a guide how to get `./go test` running properly on Mac OSX would be really helpful. I can then start writing some tests against it. 

Done CLA.