Introduce the revised coding style.

Opinion time!

I did a general revising of the coding style, since the one on the wiki is quite old. Also this will put it in a file within the repo, which is good because whenever someone makes a pull request it will have a little notification that tells people to make sure that they've read the guidelines (see [here](https://github.com/blog/1184-contributing-guidelines) for more on that). Having it in the repo will be much easier for potential contributors to see instead of the wiki.

This is the first 'draft' so I strongly encourage people to give me opinions and suggestions on this until we agree upon things. 
What about statics? I'm of the opinion we should use g_ for them too - the distinction between global and static is very small and in both cases we're talking about data that can be accessed from several different functions.