OmniAuth strategy to login with Auth0

An OmniAuth strategy for authenticating with Auth0. This strategy is based on the OmniAuth OAuth2 strategy.warning Important security note for v2: This solution uses a 3rd party library that had a security issue(s) in v2. Please review the details of the vulnerability, including Auth0 and other recommended mitigations, before implementing the solution in v2. Upgrading to v3 of this library resolves the issue.