added sizes on docker ps and docker images

Fixes #22


## containers

display size of rw and if available size of rootfs, in this exemple, only the last one is running:

    ./bin/docker ps -a             
    ID             IMAGE         COMMAND                CREATED        STATUS         SIZE
    1467f91a2709   base:latest   /bin/bash              30 hours ago   Exit 0         16K
    25b1a4d9d238   base:latest   /bin/bash              30 hours ago   Exit -127      16K (virtual 4K)
    889eed10d120   base:latest   /bin/bash              30 hours ago   Exit 0         16K
    06776824bc28   base:latest   /bin/bash              30 hours ago   Exit 0         16K
    6fb4de5c4bea   base:latest   /bin/bash              30 hours ago   Exit 0         16K
    019bac054a0c   base:latest   /bin/bash              31 hours ago   Exit 0         4K (virtual 4K)
    77530940f940   base:latest   /bin/bash              31 hours ago   Exit 0         4K (virtual 4K)
    1cae820e12ca   base:latest   /bin/bash              31 hours ago   Exit 0         4K (virtual 4K)
    e295719906dd   base:latest   /bin/bash              31 hours ago   Exit 0         16K
    a6190ecabe37   base:latest   /bin/bash              31 hours ago   Exit 0         4K (virtual 4K)
    1650aa02482b   base:latest   /bin/bash              31 hours ago   Exit 0         4K (virtual 4K)
    c65e9a2ea3c0   base:latest   /bin/bash              31 hours ago   Exit 0         4K (virtual 4K)
    a3f224ff5229   base:latest   /bin/bash              32 hours ago   Exit 0         16K
    9ab960de2f8a   base:latest   /bin/bash              32 hours ago   Exit 0         16K
    bb8f8d1b2ea0   base:latest   /bin/bash              4 days ago     Exit -127      16K (virtual 4K)
    2d358ce51d87   base:latest   /bin/bash              4 days ago     Exit 0         16K
    a13f4ba6204a   base:latest   /bin/bash              4 days ago     Exit 0         16K
    59bd80f6f359   base:latest   /bin/bash              4 days ago     Exit 0         16K
    65559fd8e242   base:latest   -i -t /bin/bash        4 days ago     Exit 127       16K
    a18f1dfef7ca   base:latest   /bin/sh -c while tru   4 days ago     Up 1 seconds   16K (virtual 171M)


## images

display size of current layer, and parent's layers as virtual

    ./bin/docker images           
    REPOSITORY          TAG                 ID                  CREATED             SIZE
    dhrp/sshd           latest              1bd442970c79        2 weeks ago         20K (virtual 284M)
    base                ubuntu-quantal      b750fe79269d        3 weeks ago         24K (virtual 171M)
    base                ubuntu-12.10        b750fe79269d        3 weeks ago         24K (virtual 171M)
    base                ubuntu-quantl       b750fe79269d        3 weeks ago         24K (virtual 171M)
    base                latest              b750fe79269d        3 weeks ago         24K (virtual 171M)
    <none>              <none>              fa594d99163a        2 weeks ago