KNX autorefresh

Added a feature which will allow to configure KNX items as auto refresh items (issue #1456). Some devices can only be pulled and don't send updates by themselves. This feature allows for such polling.
An optional parameter in the KNX binding configuration string controls whether an item should be reread and how often that reread should occur.
Previous syntax:
`knx="[<][<dptId>:]<mainGA>[[+[<]<listeningGA>]+[<]<listeningGA>..], [<][<dptId>:]<mainGA>[[+[<]<listeningGA>]+[<]<listeningGA>..]"`
New syntax:
`knx="[<[(autoRefreshInSecs)]][<dptId>:]<mainGA>[[+[<[(autoRefreshInSecs)]]<listeningGA>]+[<[(autoRefreshInSecs)]]<listeningGA>..], [<][<dptId>:]<mainGA>[[+[<[(autoRefreshInSecs)]]<listeningGA>]+[<[(autoRefreshInSecs)]]<listeningGA>..]"`
Examples:
`knx="<(5)1/1/10"` will cause a reread of the main GA 1/1/10 every 5 seconds
`knx="1/1/10+<(60)0/1/13+0/1/14+0/1/15"` will cause a reread of the listening GA 0/1/13 every 60 seconds
`knx="<1/1/10"` will cause an initial one time read (initialization) of the main GA 1/1/10