Added support for minimal async client capable of pipelined request execution

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpasyncclient/trunk@1630967 13f79535-47bb-0310-9956-ffa450edef68
Fixed minimal async client integration tests

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpasyncclient/trunk@1630968 13f79535-47bb-0310-9956-ffa450edef68