Make hash.txt optional with a warning message

[zbateson]

@armenak --

The static loading for hash.txt in Core.java spits out an ugly error message even if not using encryption methods. I propose we change it so:

1. A small warning message appears instead without a stack trace
2. An error is shown when calling 'getHash' if getHash is null, and an exception is thrown at that point

If that looks okay to you, feel free to merge :)

Thanks, Zaahid

[armenak]

Thank you Zaahid!