Made the Zeroconf project a multi target project and add a net46 target

[spudwebb]

We need to target 4.6 exactly because this is what System.Reactive is targeting, if we target 4.6.1 or 4.6.2 we end up with a ton of additional runtime assemblies to the output dir

[clairernovotny]

Just a head's up that Rx close to releasing 4.2 and that will require 4.6.1 as a minimum.

[spudwebb]

Just a head's up that Rx close to releasing 4.2 and that will require 4.6.1 as a minimum.

then I guess we will have to change the target to 4.6.1 in Zeroconf once it references Rx 4.2