JWT: Removing opinionated usage of Claims

[cam-stitt]

I created a PR to introduce support for additional claim objects in #488. After doing some thinking I came to a conclusion that some more work needs to be done on the jwt package.

• Remove the ambiguous Claims object
• Enforce the Sign/Parse methods to accept a jwt.Claims parameter

The current implementation leans towards only using MapClaims and I'm not sure what value the go-kit Claims object is providing.

[vgheri]

Agree, I'm not using the go-kit jwt middleware for this exact same reason.

[cam-stitt]

I ended up just modifying #488 to completely remove the ambiguous Claims type.

The other major change is that this is a backwards incompatible change.

[basvanbeek]

#488 has been merged and closes this issue