Professionalized

Did the following:
closed all resources in finally blocks
checked whether delete calls actually worked before advancing
logged all errors in error text files
stopped methods after error is received
made Downloader constructors both call constructor method which reduces
total code in file
allowed property "user.dir" to be customizable
allowed URL to be pinged to test for internet connection to be
customizable
slightly improved readability with && instead of as many nested ifs
added comments (not very good, though!)
allowed downloads to be cancellable on the fly and the main file is not
lost