This patch does the following major changes:
1. Add a method to load all the inodes;
2. Add a method to load one inode except for its name;
3. Make the ImageLoader constructor to take fsnamesystem as a parameter.
Some subtle (and controversial) changes made are:
1. When a child inode adds to namespace (fsdirectory), its parent inode is always known.
2. When loading an inode, BlockInfos are created instead of Blocks.