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)
Path to a logo image is defined on a per-service level and defined in the service registry. We provide a sample image for the most generic service definition but of course it could be configured to be more specific per service id.