Jay.Chang

This PR addressed issue #50 , but I don't think I fully understand the reason why that if we not initial the disk to zero, it will cause error happended.

Input/output error

5

If I fill the image with `/dev/random`, command `touch` or `mkdir` will error ``` $ dd if=/dev/random of=test.img bs=1M count=32 ``` and then ``` $ touch test.txt ``` the error...

I discover that deleting a file in the current system has a high overhead. When a file is deleted, to maintain the compactness of simplefs_dir_block, subsequent files are shifted forward...