Allow for RS256 Token Signing

[andrew-womeldorf]

- Do you want to request a feature or report a bug?
Feature.

- What is the current behavior?
HS256 is the only supported signing method.

- If the current behavior is a bug, please provide the steps to reproduce.

- What is the expected behavior?
Allow for specifying other signing methods, including RS256.

- Please mention your Go version, and operating system version.
go version go1.11.4 linux/amd64