Fix hex encoding of message to create valid hex strings

[danfinlay]

When a non-hex message is submitted for signing, it needs to be encoded into hex in a reliable way.

The previous method did not create valid hex strings, they had an odd length, so this new method pads the hex string to ensure validity.

[danfinlay]

While this does show your message if you don't pre-hex-encode your message, since the message that is ultimately signed is binary, if you don't provide your own binary data to sign, it can be difficult to recover that data.

We should provide some good sample code for recovering from a text message.

Or maybe that's what personal_ecRecover is best for...

[kumavis]

couple issues - accidentally included unrelated file

has some custom hex encoding code?

test data is incorrect