Rewrite JWT code using new JWT/JWS classes introduced in Winter 17

http://docs.releasenotes.salesforce.com/en-us/winter17/release-notes/rn_apex_new_classes_methods.htm#rn_apex_new_classes_methods

Winter '17 introduces JWT and JWS classes to handle JSON Web Token interactions. Once Winter '17 has been deployed to all orgs, that code should be rewritten to take advantage of these methods rather than being custom implementations.

Start with the linked Winter '17 release notes to understand the JWT and JWS classes, then locate the SDK's custom JWT implementation. Confirm Winter '17 availability in all orgs before changing it; done means the custom implementation has been replaced and JWT interactions still work through the new classes.

Written by the indexing model from the issue text.