Add network statistics gathering to the LocalJmxMonitor

Added new threads to capture the number of inboud and outbound bytes for
an interface. (NetworkBytesMonitor and subclasses NetworkBytesInMonitor
and NetworkBytesOutMonitor) The /proc/net/dev code was shamelessly
stolen from Michal's SimpleNetworkTrafficCollector SmartFrog class.

Add references to the new threads in LocalJmxMonitor, and only enable
them when the interface name field is set.

Add interface name property to StartJVMMonitorStage and set the field in
LocalJmxMonitor when the property is specified.

Plot the network statistics in LocalSystemMonitorChart
Update network stats collection code

Fixed a bug in LocalJmxMonitor that prevented bytes transmitted from
being collected. Added comments to new classes, and updated the XSD.
Updates from Radim's comments

Deleted NetworkBytes[In|Out]Monitor classes, and replaced them with a
factory method in NetworkBytesMonitor.

Updated LocalJmxMonitor to use the factory method.

Reworked CSV reports in LocalSystemMonitorChart
Avoid NPE if no network statistics were collected
Updated XSD, polished network bytes monitor creation