Add Swift Package Manager support

[luisdelarosa]

Since this is a Swift library, adding Swift Package Manager support will make it easy for other apps to utilize it.

[PasiSalenius]

@luisdelarosa Should the package be updated with a new version, now that I committed an update that changed sending packets from a synchronous operation to async? Would, for example, version 1.1 be appropriate here? Not that familiar with Swift Package Manager myself.