Add a method for setting the proxy

    /**
     * Sets the Proxy
     * @param hostname  the hostname (IP or DNS我· name) 
     * @param port  the port number. -1 indicates the scheme default port.  
     */
    public void setProxy(String hostname, int port){
        final HttpHost proxy = new HttpHost(hostname, port);
        final HttpParams httpParams = this.httpClient.getParams();
        httpParams.setParameter(ConnRoutePNames.DEFAULT_PROXY, proxy);
    }

Usage:
AsyncHttpClient client = new AsyncHttpClient();
client.setProxy("10.0.0.172",80);
client.get("http://www.google.com", new AsyncHttpResponseHandler() {
    @Override
    public void onSuccess(String response) {
        System.out.println(response);
    }
});