Waf generator and some tests

[AlexRamallo]

Hi,

I've had this custom Waf generator floating around some of my projects for a while, and while it was buggy, I was using extra layers in my stack to work around any issues.

However, I got motivated a few weeks ago to finally fix it and try to make it more standard, and right now I think it's in a much more useful state as a standalone generator. So I'm posting it here after @memsharded's recommendation in case someone else using Waf can try it out and report back.

One part I'm not too sure about in this PR is the tests, since I also had to include a copy of the waf script in there. Maybe it'd be better to have the test script download it instead, since most people who config install probably won't need it?

Usage

The generator works by generating a waf tool, which is essentially just a Python script that the user loads during project configuration. This means the generator has a big ugly string template in it, but it makes it much easier to use since it means it can be added to any existing Waf project without having to install/ship a custom Conan extension (and worry about versioning issues).

I also wrote some guides on how to use it on my website