MDL-33032 Fixed issues for file integration

MDL-33032 Fixed issues for file integration

1. Added tracker issue number in comment
2. Fixed stored_file::delete_reference()
3. repository::send_file() will throw exception if not implemented by subclass
4. Fixed renaming unit test, added one unit test for deleting original file
5. Fixed copyright statement for googledoc and picasa repository plugin
6. Implemented stored_file::set_filesize()