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)

This is done now in the most recent commits.