Ishimotto - Adding AriaDownloader

Finally I finished to add the AriaDownloader to Ishimotto and it works fine :)

I also removed redundant methods from the Program.cs, and add FileWriter class to help the Aria process.

In Addition I logged all activities of Ishimotto and built Unit Tests to test functionality. 

2 things I'm not proud about but I couldn't find better solution:

1. In IshimottoConfig.cs you can see that I turn to a text file to find out when were the program was last executed (line 95).
I do it so I can update the text file in the end of the process and I couldn't find better solution (create DB to save 1 detail is not a solution ...)

2. When i built the unit tests I encounter a strange thing when static methods like File.Create or Directory.Create were holding an handle to the file / directory when I was trying to delete it during TrarDown.
I came with an Retry mechanism, but perhaps you know what is the reason for the handle.

Thanks,
Tomer K