Add a setting for network polling interval

The polling process may take many seconds to complete if a large filesystem is connected over a relatively slow link.  If the amount of time required to do a single pass is longer than the polling update interval then the CPU load may remain pegged at 100% because XBMC starts a new update as soon as the previous one has finished.
    
I have added a new setting called POLLING_INTERVAL which can be used to configure the amount of time (in seconds) between successive polls. This setting only applies to the XBMCVFS based polling methods.