provide interface that exposes ahc's interface for decoding the str response

The `as.String` response handling function will currently return the request body string decoded with [the default charset](https://github.com/AsyncHttpClient/async-http-client/blob/async-http-client-1.8.1/src/main/java/com/ning/http/client/providers/netty/NettyResponse.java#L87-L89) defined in the netty provider, [ISO-8859-1](https://github.com/AsyncHttpClient/async-http-client/blob/async-http-client-1.8.1/src/main/java/com/ning/http/client/providers/netty/NettyResponse.java#L87-L89). ACH does however  provide an interface for [decoding the response in a specified charset](https://github.com/AsyncHttpClient/async-http-client/blob/async-http-client-1.8.1/src/main/java/com/ning/http/client/providers/netty/NettyResponse.java#L91-L93).

This change set exposes that interface in dispatches `as.String` module by adding `as.String.charset(java.nio.charset.Charset)` and for convenience, `as.String.utf8` for common modern day response encodings. 
Ah, I see it [now](https://github.com/AsyncHttpClient/async-http-client/blob/async-http-client-1.8.1/src/main/java/com/ning/http/client/providers/netty/NettyResponse.java#L133)