Fixed folder structures:

• renamed FSharpDoThis to DoThis in Unit 3
• removed the CSharp folder from Unit 1, moved the content of the FSharp folder one level higher

Fixed all dead links to the Akka.NET

• Got broken after the update to the new docs website.
• The only link I couldn't find in the new docs is the API reference for ForkJoinDispatcher (old link: http://api.getakka.net/docs/stable/html/F0DC1571.htm). I replaced it with this instead: http://getakka.net/articles/actors/dispatchers.html#forkjoindispatcher