Logging update for EpidemicServer

Previously `EpidemicServer` used a lot of calls that looked like this:

```java
if (DebugReceive) {
    debugMessage("accept diff " + origin + " " + startSequenceNumber + " "
        + endSequenceNumber + " head " + head + " body " + body);
}
```

Where `DebugReceive` was a static final boolean set at the top of the file.  Continuing the work done as part of Issue #7 and the associated discussion, this replaces all of those calls with the following pattern:

```java
debugMessage(DEBUG_RECEIVE, "accept diff {} {} {} head {} body {}",
    origin, startSequenceNumber, endSequenceNumber, head, body);
```

Where `DEBUG_RECEIVE` is a [slf4j marker](http://slf4j.org/api/org/slf4j/Marker.html).  This way changes can be done at run/launch time, instead of requiring a recompile of the client, while still keeping the filtering capability and expressive power that the static final variables provided (improving it somewhat since this information is then communicated in the logging message). 

As part of this update a logback configuration has been included as well that, by default, uses the standard console logger but only logs messages level `info` and up.