Added `/apps.json` Pow endpoint

I wanted to get information about Pow's currently running applications, so I added another REST endpoint. I needed to add `toJSON()` to `RackApplication` to share out information about a running application. I added a `runningApps()` method to `HttpServer` to collect all of its `RackApplications` and package their `toJSON()` output for the endpoint. I added a test to cover `HttpServer`'s new endpoint. I added documentation for this new endpoint to the manual. Finally, I bumped the version to `0.4.4` with support for this new endpoint. Let me know if there's more I can do for this feature.