First part of CMIS-874 (Switch from NSURLConnection to NSURLSession).

The CMISHttpXXX classes have been moved to use NSURLSession but they currently always use data tasks. To support background upload and download operations the appropriate task has to be used. A different session configuration object is also required to run in the background, the 2nd part will address these shortcomings.

git-svn-id: https://svn.apache.org/repos/asf/chemistry/objectivecmis/trunk@1644593 13f79535-47bb-0310-9956-ffa450edef68