Nix support

This is a superset of #14 , so closing this will close both.

It simply add a default.nix targeting the new iCalendar release.
I'm pretty new to the whole nix business, perhaps @ocharles could doublecheck this?
It's a run-of-the-mill nix file generated by cabal2nix, but the "networkUri" dep was made explicit as apparently cabal2nix doesn't recognise flags (at least disabled ones) in a cabal manifest.

Alfredo