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)
I am not sure I follow you here. We do test to see if the logo is defined; are you saying we should check and see if the logo actually does exist at the specified path? and if it doesn't, print a default logo? 

I'd be fine with those changes, but I think it's a bit overkill. Only reason a logo would not show up is because it doesn't exist at the path specified, or it's not specified at all. My analysis would be:

1. If there is anything in the logo, use it. (if it exists it will be shown, it not, it wont)
2. If there is nothing, fall back onto a default image.

That way, we leave the burden of checking for the path and diagnosing the logo issues over to adopters. otherwise we'd have to add scriptlets here to the login page. logo could also be a URL as well, which would be tricky because it can be dynamically generated. those are not easy to check correctly.