Port to CMake

Hi,
I ported Wslay to use CMake as build system. I changed the API a little. You have to include wslay.h using `#include <wslay.h>` not `#include <wslay/wslay.h>`. I consider to have wslay.h in special directory unnecessary, because there will be only one .h file. I also fixed some compile warnings and I ported patch from https://github.com/benvanik (it should be possible to compile Wslay at Windows). I tested this build system only at Linux. I didn't do any changes of indentation.

Jiri