Pass in keys as byte arrays instead of strings

[raboof]

Right now, at least for HMac, the String key is converted to Array[Byte] (https://github.com/jasongoodwin/authentikat-jwt/blob/master/src/main/scala/authentikat/jwt/JsonWebSignature.scala#L47) using String.asBytes (http://docs.oracle.com/javase/8/docs/api/java/lang/String.html#getBytes--).

This is kind of confusing, and unless I'm missing something is platform-dependent and reduces the key space to only those keys that are valid strings in the default encoding of the platform.

[jasongoodwin]

I'll try to look at this item this week. Thanks for the feedback.

[xissy]

Can you please check and merge the PR?