Improve JanusGraph server documentation

Several [questions listed here](https://groups.google.com/d/msg/janusgraph-users/FuezPYbo8lM/iHuIXoJqBwAJ) from the users mailing list can be added to the docs to help on-board new users.

1) What is the relation between Gremlin server (bin/gremlin-server.bat) and the JanusGraph server (bin/janusgraph.sh)?

2) I've specified my Cassandra related configuration values in conf/gremlin-server/janusgraph-cassandra-es-server.properties file and this file is being used when running the gremlin server. While using the Java API (from Scala), I do the following -

```
val graph: JanusGraph = JanusGraphFactory.build().
set("storage.backend", "cassandra").
set("storage.hostname", "localhost").
set("storage.cassandra.keyspace", "MyJG").
set("storage.username", "username").
set("storage.password", "password").
open()
```

Should I be using the same (conf/gremlin-server/janusgraph-cassandra-es-server.properties) file which I use to start the gremlin server from my Java code?

3) In the above API, I haven't specified the JanusGraph server endpoint (the URL or the port), so which server is my Java code connecting to?

4) Does Java API use websockets, and can JanusGraph server run on a different machine (right now, my Cassandra and gremlin server run on the same machine)?

5) Is Java API the same as Gremlin language / API?

6) Where is the documentation / examples for REST API (for adding / querying vertices, edges)?

7) How can one achieve graph namespacing? So for example, I have to create three different graphs for employees, vehicles and cities, how can I segregate the data for these three graphs? Can I give a name / id to the graph? Or do these graphs have to be stored in different Cassandra keyspaces?

8) If the graphs have to be stored in different Cassandra keyspaces, how can I connect to these different graphs / keyspaces from the same Java application?

Start with the eight questions in the issue, the linked users mailing-list discussion, and the referenced bin/gremlin-server.bat, bin/janusgraph.sh, and conf/gremlin-server/janusgraph-cassandra-es-server.properties paths. Done means the server relationship, configuration and Java API behavior, REST examples, and graph namespacing guidance are documented clearly for new users.

Written by the indexing model from the issue text.