Display in hex via helper class.

[JMarkstrom]

Added:

1. Ability to provide secret in hex when programming
2. Display of secret in hex (instead of byte)
3. Helper class for hex encode/decode.

[virot]

Should we also move from:
Converter.StringToByteArray
Converter.ByteArrayToString

Yours are much cleaner

[JMarkstrom]

Ah, but you also bring my attention to the fact we already had a converter.cs. So should base32 and hex go in there? Your call.