Support array of strings and single string for aud value

[dklesev]

Fix #348

[simonbrynych]

@dgrijalva don't you mind to merge one of fixes for audience as a []string, or string (as RFC says). This one looks like the best one to me.

[leucos]

@dgrijalva do you plan to merge this PR ?

[enj]

Using interface{} here is the wrong approach. A custom type Audience []string with the appropriate unmarshall implementation will give you type safety and avoid panics.

Any attempt to fix this will likely be backwards incompatible from a Go perspective.

[dgrijalva]

This is a backward incompatible change. Please see the 4.0.0-preview1 release and see if that resolves your issue.