Add a convenient overloaded constructor so that we can use NingWSClient in a few keystrokes

This PR saves a few characters when you want to use the HTTP client without the hassle of starting a Play application.

Instead of writing the following:

```scala
val client = new NingWSClient(new NingAsyncHttpClientConfigBuilder(DefaultWSClientConfig()).build())
```

You can now just write this:

```scala
val client = new NingWSClient(DefaultWSClientConfig())
```