Fix for strict standards issue

Due to strict standards being default error-level in php5.4+, there was an issue with the getRequest-method in the specific Integration-implementations (Admin, Verify etc.). 
This fix removes class-inheritance from the Integration base-class, as the inheritance seemed obsolete, and to avoid the strict standards issue.

Additionally, according to the documentation, the [Verify-api](https://www.duosecurity.com/docs/duoverify#authentication) also requires the date-parameter to be set for authentication.