making FileOverwriterOutputWriter.java backwards cmpatible.  It now look...

...s for the <showTimeStamp> optional tag in configuration.  Defaults to false.

When config looks like this:
```
<outputWriter class="org.jmxtrans.agent.FileOverwriterOutputWriter">
      <fileName>jmxOutputFile</fileName>
      <showTimeStamp>true</showTimeStamp>
</outputWriter>
```
Output looks like this:
```
[2014-10-27T13:01Z] jvm.heapMemoryUsage.committed 582483968
[2014-10-27T13:01Z] jvm.nonHeapMemoryUsage.used 29967688
[2014-10-27T13:01Z] jvm.nonHeapMemoryUsage.committed 43122688
[2014-10-27T13:01Z] jvm.loadedClasses 4098
[2014-10-27T13:01Z] jvm.thread 32
[2014-10-27T13:01Z] tomcat.requestCount 8
[2014-10-27T13:01Z] tomcat.requestCount 0
[2014-10-27T13:01Z] tomcat.errorCount 0
[2014-10-27T13:01Z] tomcat.errorCount 0
[2014-10-27T13:01Z] tomcat.processingTime 545
[2014-10-27T13:01Z] tomcat.processingTime 0
[2014-10-27T13:01Z] tomcat.bytesSent 47373
[2014-10-27T13:01Z] tomcat.bytesSent 0
[2014-10-27T13:01Z] tomcat.bytesReceived 0
[2014-10-27T13:01Z] tomcat.bytesReceived 0
[2014-10-27T13:01Z] application.activeSessions 0
```

When config looks like this:

```
<outputWriter class="org.jmxtrans.agent.FileOverwriterOutputWriter">
      <fileName>jmxOutputFile</fileName>
</outputWriter>
```

Output looks like this:
```
os.systemLoadAverage 1.2041015625
jvm.heapMemoryUsage.used 78076928
jvm.heapMemoryUsage.committed 593494016
jvm.nonHeapMemoryUsage.used 30075208
jvm.nonHeapMemoryUsage.committed 44236800
jvm.loadedClasses 4095
jvm.thread 26
tomcat.requestCount 1
tomcat.requestCount 0
tomcat.errorCount 0
tomcat.errorCount 0
tomcat.processingTime 530
tomcat.processingTime 0
tomcat.bytesSent 11398
tomcat.bytesSent 0
tomcat.bytesReceived 0
tomcat.bytesReceived 0
```

Should be 100% backwards compatible.