Add support for checking against multiple issuers

[jquense]

👋 hey there. This is a compliment to PR #306 it adds support for checking against multiple issuers. Our use-case is to enabling rolling migrations from one auth provider to another, where the audience but also the issuer has changed, without breaking verification for already issued tokens.

cc @yodaiken

[coveralls]

Coverage remained the same at 100.0% when pulling 6469397 on jquense:add-iss-as-list into c1253ec on jpadilla:master.

[coveralls]

Coverage remained the same at 100.0% when pulling fa9b5b2 on jquense:add-iss-as-list into c1253ec on jpadilla:master.

[taion]

Any update here? This seems like a pretty innocuous change, and it would simplify life for us.

[jpadilla]

LGTM. We still need to update docs and changelog.

[coveralls]

Coverage remained the same at 100.0% when pulling 2a659cb on jquense:add-iss-as-list into e0aa10e on jpadilla:master.