implemented basic video interface

* ioutil.asm: renamed outseq to output_sequenve for clarity
* main.asm: added SP initialization
* video.asm: added a lot of code (loosely inspired by the GD library)
Hello, world!

* video.asm: hide sprites on reset
* main.asm: now prints "Hello, world!" on screen :)
Merge branch 'video'

The basic video interface is working now, so it can be included in master.