Added Node-level last-update reporting feature.  Whenever a node's sensor sends a variable update to the gateway, the plugin updates a node-level LAST_UPDATE timestamp for that node that is displayed in a human-readable HH:MM am/pm form on the node's dashboard and Settings tab.
Time format is set based upon Metric/Imperial plug-in setting.
Merge pull request #3 from bblacey/last-update

Added Node-level last-update reporting feature.  Whenever a node's senso...