Support `additionalInformation` on clients

The `additionalInformation` map from [BaseClientDetails][0] can be used to add arbitrary data to a client object. This change supports reading `additionalInformation` blocks from clients defined in a config file.

In my project, we use `additionalInformation` to store a human-readable name and metadata on how the client should be handled.

I'm not aware of any problems from parsing `additionalInformation` in the config file, so I thought I'd submit this PR!

[0]: http://docs.spring.io/spring-security/oauth/apidocs/org/springframework/security/oauth2/provider/BaseClientDetails.html#getAdditionalInformation()