Running individual HealthChecks and getting registered names

I'm trying to create a website for checking health status of our internal projects similar to the one Mozilla has set up for their own services: http://status.mozilla.com/

I would like to be able to make a separate request for each of the checks in order to make it naturally asynchronous. But in order to do that, I need to be able to run individual health checks via HealthChecks interface, like:

```java
    public static Result runHealthCheck(String name) {
        return DEFAULT_REGISTRY.runHealthCheck(String name);
    }
```

Moreover, it would be great to be able to get all registered HealthCheck names:

```java
    public static Set<String> getHealthCheckNames() {
        return DEFAULT_REGISTRY.getRegisteredNames();
    }
```

It's a trivial change that would help me a lot.