CAS-1341: Offer the option to provide a new HTTP client implementation

Let's take an example :
I have a CAS server with internal applications and internet applications integrated.
For logout process, I use back-channel HTTP communications which are different according to the services (direct communications for internal services and use of a proxy for internet applications).
I would need a specific HTTP client implementation.

The main idea is now to have an interface : ```HttpClient``` and a default implementation (sufficient in almost all cases) : ```SimpleHttpClient``` (the old HttpClient renamed).