* fix bug in audience check while verifying JWT
- previously the check was converting the audience type into a string
and then comparing with the conf value. all audience types (as it is a
string or URI) will convert to plain strings
- use the Audience type from the jose library for comparing
* add docs for audience
* add issuer check as well
* docs minor syntax fix
* skip audience check if not given in conf
* minor docs update
* qualify import jose library
