Java beans for REST service

Added a couple Java beans (or POJOs, whatever you want to call them) for the REST service.
 - `LoginBean.java` for logging in to M14, REST will append the path `/login`. Contains a `username` and `password`. Formerly known as `UserPassBean.java` and updated to a name that actually makes sense.
 - `MpdClient.java` for creating and destroying connections. REST will append and utilize the paths `/create` and `/destroy` and use this bean to create and destroy connections in the database, as well as establish and remove edges between the users with the provided username. Contains the MPD `host` and `port`, and a `username` belonging to the user the connection is attached to.