Improved MenuSelection and implemented the CurrencySelection

I made the userexperience a bit more "enjoyable" so you can for example reenter the amount without getting back to the MenuSelection and starting all over again. Also added javadoc documentation.

Implemented the CurrencySelection (with your help guys, thanks again) and it seems to work but I can only say for sure, when the CurrencyResource.txt is up and loadable.

On a sidenote I have to try the clearConsole() methode with an Linux os but it should work.

Edit: Did some fine tuning so when the user enters 0, the user will get a proper error message and changed the rate in the Currency file from double to Double