cmakeize project

I put the source in src/main/{c,include} for the library sources and src/test/{c,include} for test sources.

I used cmake to generate the makefile (more flexible than a classic makefile). It is more flexible and will allow to add automated testing easily. 

To build you have to create a folder and do `cmake /path/to/my/project/folder`, then `make`. Doing `cmake .` in the same folder as the project would work I think but I prefer a build folder separated from the sources. I updated the readme according to the changes.

I also put the images used in the doc in doc/images, to avoid having generated content in the same folder.

There is no changes in the code itself, only the organization of directories.