glfw added

# Features
* remove xorg
* add glffw. you need libglfw-dev, libglew-dev
* Makefile for only C. something like makefile for VS is not yet (because you don't use windows)

* void sleepMicrosecs(unsigned long useconds);
    * function like usleep for cross platform
* bool isWindowOpened();
    * for glfw

tested on linux(on my virtual machine), RPi, RPi without X. and windows.
I test only examples/simple.c.

# TODO
screen width/height for GLFW? your xorg version is hard-coded.
```
__display_width=640;
__display_height=480;  // xorg hard coded for now
```

how do you think about changeing ```int makeContext();``` to ```int makeContext(int w, int h)```? if RPi/RPi without X, ignore w,h. else, use w,h as display width/height.