Add eitherDecodeClaims

[Lucsanszky]

As discussed here: #35 , this PR adds a utility function which preserves the error messages when decoding claims. @tekul, I was bold enough to add a String parameter to the BadClaims error. Let me know if this is acceptable or you would rather see me introduce a new error constructor.

[tekul]

Added some comments. On reflection I think it's overkill having the extra function, since the original one is exactly the same (just with a fixed error message). Let me know what you think.

[tekul]

This should probably just be something like:

Nested _ -> throwE (BadClaims "A JWS can't contain a nested JWT")

since that's why the error is being reported. It would be a coding error rather than a client issue so not something that should happen in practice. I don't think there's a need to log the encode content.

[tekul]

If we are going to add an error message to BadClaims then I guess that is a breaking change for existing code which pattern matches on the error, so I'd prefer to just modify the existing decode function here to record the error, not bother with the extra eitherDecodeClaims version (which has the same signature) and just bump the library version to 1.0.

[Lucsanszky]

Thanks for your review! I added the changes that you have requested, let me know if this looks any good to you.