Support for displaying RegisteredService information on login form

Relates to https://github.com/Jasig/cas/issues/539

This pull simply just adds a `logo` field to the `RegisteredService` class, and adds the retrieved registered service instance into the UI context. The UI is then decorated with the logo path and service name and description to display that info when a service request is found. 

Tested with the JSON service registry and worked just fine. 

Sample screenshots:
![](http://i.imgur.com/k52316a.png)

Mobile view:

![](http://i.imgur.com/KsBLQBq.png)
No, I don't think we should test if the logo actually exists on disk or wherever it is stored.

We should just test if the logo property is not empty: in that case, use it and display the defined logo. Otherwise, we should use the default logo.
I would always display the name of the service and the description if it is not empty.

Does it make more sense?