Created fd.img

fd.img is a floppy disk image with GRUB 0.96 pre-installed, formatted as ext2.
Updated fd.img

created menu.lst in the floppy image
Added *.img and *.iso to .gitignore

Disk images are used to test the OS with virtual machine, so disk image would change significantly between versions. Besides, disk images are binary files, making it difficult for git to keep track of its diff.
Created link.lds as the ld linker script.

link.lds is used in the linking procedure to produce the kernel image. The kernel image is formatted in flat binary with 3 sections: `.text` (with bootstrap code), `.data` (ro and rw data), and `.bss`.
Added Makefile and a simple boot stub.

Current makefile can compile and link the kernel image, write the kernel image to a floppy disk image (needs root privilege), and cleanup things.
Maybe in the future the build system would change to CMake.
Replaced Makefile with a brand new one

This version of Makefile could find source files in the directories and generate rules for them, and place object files in a separate directory.
Added long mode setup code

multiboot.asm now contains codes for setup and entering a minimal 64-bit environment, which does not support interruption, address space projection, etc. C code will be compiled and run in 64-bit mode.
Changes in naming conventions.
Merge branch 'develop'

The OS can boot with grub and enter long mode.