- Improve startup time.

- Cleanup placeholders.

- Add support to download rootfs.

- Fix:

    $ kains -B / /tmp/whatever kains -r /tmp/whatever
    Error: mount("/", "/tmp/whatever/.kains-20551", "", 0x45000, ""): No such file or directory

- Add comparison with PRoot:

  ======================================  ==============  ==============
                                          kains                proot
  ======================================  ==============  ==============
  Linux                                   >= 3.8.0        >= 2.6.0
  has an impact on performance            no              yes
  needs root privileges                   no              no
  grants root privileges                  partially       partially
  preserves auxiliary groups              no              yes
  support ptrace                          yes             emulation
  can bind symlinks without XXX           no              yes
  can bind to non existing path           no              yes
  ======================================  ==============  ==============
