HTTPASYNC-58: upgraded HttpClient to version 4.3.1

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpasyncclient/trunk@1533205 13f79535-47bb-0310-9956-ffa450edef68
Updated HttpAsyncClient 4.0 release notes; updated building instructions

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpasyncclient/trunk@1533206 13f79535-47bb-0310-9956-ffa450edef68