adding support for custom drivers

After reviewing the code that I wrote yesterday, it is clear that while the changes I proposed do allow the "built in" cache drivers to utilize the graceful cache, it does not address your original elasticache use case which involves manually creating a new GracefulCache\Repository in your driver. 

So, the changes I proposed are good for using your class to help with built in drivers, but doesn't really help your elasticache use case. 

I have added a bit more code and some documentation in the README to help address this. The CacheManager child class overrides the parent class method for creating the Repository object which is protected. I added a public method in the child class to permit access to this method. 

Take a look at the README and the code changes. Let me know how that goes.