Add rudimentary tests for Auth0ServletCallback.init(ServletConfig):
- failOnMissingInitParameters: passes
- failOnMissingServletContextParameters: fails
Read and validate parameters consistently in the following order:
1. ServletConfig.getInitParameter(String)
2. ServletConfig.getServletContext.().getInitParameter(String)

Toss cumbersome, unused and thus broken "validation" logic for:
"auth0.client_id", "auth0.client_secret", "auth0.domain"
Rewrite Auth0ServletCallbackTest to test validartion on all parameters
individually (for both ServletConfig and ServletContext)