CAS-1316: Refactored the way SLO messages are sent by the HttpClient

 The current SimpleHttpClient acts as a generic impl and yet somehow is tied to sending logout requests via the inner `MessageSender` class. This pull changes that behavior with the following:

1. `MessageSender` becomes its own class, `CallableMessageSender`
2. `HttpClient` is instead refactored to accept a message sender, as opposed to bits and pieces of the message itself.
3. `SimpleHttpClient` by default detaches itself from SLO and uses the `CallableMessageSender`
4. `LogoutManagerImpl` preps an instance of `CallableLogoutMessageSender` in order to encapsulate SLO-specific details

`CallableLogoutMessageSender` is different in the following ways:

1. Explicitly sets the content type to `application/xml`
2. Prepends the `logoutRequest`  parameter to the message sent (which preciously was hardcoded into the http client)

We really should deprecate the old method and introduce this new method (and have them set the same property)