Add first method to begin with download in background
Changes in downloads methods
Add commnets for the new methods
Add new block methods to get the finish download event and callbacks of the progress of downloads
Update AFNetworkig to version 2.3.1 and change setDownloadTaskCompleteBlock
Add a new parameter in download methods to set or not custom priority
Created a new unit test for download a file with session
Fixed the new unit test for download a file with session with the correct configuration
Merge branch 'develop' into OC-3572_background_downloads
Include the new unit test for download a not existed file
Add download with session  method in example
Task OC-4073 is complete
Remove unusued classes
Merge pull request #47 from owncloud/OC-3572_background_downloads

Oc 3572 background downloads