Expose Ning AsyncHttpClientConfig Settings

  * Add an Option[Configuration] to WSClientConfig trait to be used for client specific settings
  * DefaultWSConfigParser#parse takes Option[String] for specific client config under ws namespace.
  * NingAsyncHttpClientConfigBuilder looks for settings in "ning" ws config.

    ws.ning.allowPoolingConnection = Boolean
    ws.ning.allowSslConnectionPool = Boolean
    ws.ning.ioThreadMultiplier = Int
    ws.ning.maximumConnectionsPerHost = Int
    ws.ning.maximumConnectionsTotal = Int
    ws.ning.maximumNumberOfRedirects = Int
    ws.ning.maxRequestRetry = Int
    ws.ning.removeQueryParamsOnRedirect = Boolean
    ws.ning.requestCompressionLevel = Int
    ws.ning.useRawUrl = Boolean

Fixes #3353
Thanks for these docs! A little further up the page is some example code that shows how to build a `NingAsyncHttpClientConfigBuilder` using a `DefaultWSClientConfig`. Would you be able to update it to use a `DefaultNingWSClientConfig` instead?