Flask-of-oil

Feature:
- Refresh JWKS from JWKS URI when no suitable keys found in current JWKS.

Fix:
- Fixed verification of `aud` claim. Now properly verifies audience where a token contains a single audience value.

Feature:
- Allow configuring multiple issuers.

Fix:
- Return default errors in all cases so that they can be properly caught by flask error handlers.
- You can now omit passing required claims.

Initial release.