Add PTFileDelegate protocol, to notify file download did finished.
Aesthetic changes
Improvements

* Rename delegate method
* Call `didFinishDownloading:` delegate method either if request
finished or cancelled
Merge pull request #1 from exalted/AL333Z-master

Aesthetic changes and improvements.
Add check for delegate method invocation.
Change delegate method check. (Removed useless check)
Merge pull request #2 from AL333Z/master

Add PTFileDelegate protocol, to notify file download did finished