Add description to README.md.
Add boot.s and Makefile.

boot.s is copied directly from the [OSDev.org Bare Bones tutorial][bb].
It handles creating sections for the multiboot header, the stack, and
the code that will call into the future code for the kernel. The code
moves the stack pointer to the top of the stack and calls kernel_main,
which will be defined later. If kernel_main returns, it hangs in an
infinite loop.

[bb]: http://wiki.osdev.org/Bare_Bones

The initial Makefile includes recipes 'boot.o' and 'clean'. 'boot.o'
generates boot.o from boot.s. 'clean' removes all the non-source files
from the project.
Add main.rs.

The initial version of main.rs only includes the function kernel_main()
that's called from boot.s. It also includes a directive that tells rustc
not to link it with the standard library.

Changes Makefile to know about main.rs and how to build it.
Add linker.ld.

linker.ld is copied directly from the [OSDev.org Bare Bones
tutorial][bb]. It combines boot.o and main.o into os.bin. Also, this
change a rule to Makefile to build os.bin.

[bb]: http://wiki.osdev.org/Bare_Bones

Adds *.bin to the .gitignore file.
Add Makefile rule to run os.bin.
Add basic VGA driver.

Adds a basic VGA driver. The driver makes public two functions:
vga::init() and vga::put_char(), and one enumerated type: VGAColor. vga::init()
sets the foreground and background colors and initial cursor location
and clears the screen. vga::put_char() prints one character to the
screen and advances the cursor. The VGAColor type gives names to the 16 basic VGA
colors.

Adds calls to the put_char() function in kernel_main() to exercise the
functionality.
Update README.md with status.