Article on wrapping C libraries in Nim

[PMunch]

I know.. I was planning on waiting with the async article until I had the 
threading article done. And I was waiting with writing the threading article 
until threading in ARC/ORC was more mature since it seems like it will be a 
pretty substantial change. However there is still little clarity about 
threading with ARC, and I wanted to get the async article out because people 
kept asking questions about async. I've started writing the article on 
threading already though, so maybe I'll just publish it early..