Discover command from hdhomerun.com can return hdhomerun units on inaccessable IP addresses

[sengsational]

If the local hdhomerun_config.exe discover command is not available, the program goes out to http://ipv4-api.hdhomerun.com/discover which returns the local IP addresses and devices. If the household has multiple subnets, not all tuners will necessarily be accessible.

One solution would be to include only devices on the subnet of the machine running CwHelper.

[sengsational]

Added functionality for just after the external http discover command to filter to include only IP addresses on the same subnet (using 255.255.255.0).

I moved the discover of the machine's IP out in a separate class, "NetworkDetails", out of TunerHdhr, where it was before. Then I was able to call it from TunerManager (which is the thing that creates the tuners).

d93c140