B0rked servlet callback init logic

`Auth0ServletCallback.init(ServletConfig)` only properly validated the presence of `auth0.redirect_on_success` and `auth0.redirect_on_error`.

Also, the other parameters were read from `ServletConfig.getServletContext().getInitParameter(String)` instead of `ServletConfig.getInitParameter(String)` as with the first two.

Specifically for us this caused this code to not work together with the [Dropwizard](http://www.dropwizard.io) library.

This PR unifies parameter validation and allows parameters to be mixed across `ServletConfig` and `ServletContext` and adds a unit test for the whole shebang.