Update NHunspell using NuGet

NuGet basically manages packages for us, and I think NHunspell is a good candidate for this.
I don't know what the original problem was, but it seems that its fixed, because I don't seem to get any issue with the new version.
This also updates the native Hunspell to the latest version by affinity.

Also, I'm quite glad to get rid of an external/internal dependency. One less thing to take care of unless it breaks in which case we will see what needs fixing. (;