macOS support

[mtibben]

I'm trying to use this library on macOS, but just realised you deactivated support in #2.

Unfortunately I can't find any go library for CryptoTokenKit, so it might be easiest to get this library working. I think it should still be possible to use the PCSC API here. For example, you can see the testpcsc binary on macOS doing it here, which looks like it might be using a socket available at /var/run/pcscd.comm ?

[gballet]

Indeed, I naively thought macos would behave like FreeBSD, but it didn't so I deactivated it.

I have tried using this library on someone's mac, and indeed they have their own, incompatible version of PCSC that nonetheless gets a hold of pcscd.comm, so unless the user is willing to deactivate the default service, it won't work. I'm happy for any PR that would interface with the crypto token kit, though.