Use OS threads for watch in Windows

This fixes issues mentioned in #153, #229 and [mailing lists](https://groups.google.com/forum/#!msg/hakyll/hkEgtdSiVaw/GBEkMvNWZq8J).

We now explicitly ask people to compile their hakylls with `-threaded` option, which is necessary for unsafe FFI calls to not block all the threads.

This PR also has a downside: If OS thread creation fails for any other reason than not being compiled with `-threaded` flag it will still print the generic error message, but that should happen rarely enough to not matter IMO.