Add sizes in images and containers

Fixes #22

$>docker images -a

    REPOSITORY          TAG                 ID                  CREATED             SIZE
    busybox             latest              e9aa60c60128        7 weeks ago         6.51 MB (virtual 0 B)
    vieux               minecraft_server    6d4b7a164cc4        14 minutes ago      12.0 kB (virtual 659.55 MB)
    base                ubuntu-12.10        b750fe79269d        7 weeks ago         24.1 kB (virtual 171.75 MB)
    dhrp/mongodb-sshd   latest              59fe4e85bea3        6 weeks ago         3.364 GB (virtual 171.77 MB)
    <none>              <none>              712e29ecc84c        14 minutes ago      2.27 MB (virtual 657.28 MB)
    <none>              <none>              27cf78414709        7 weeks ago         171.75 MB (virtual 0 B)
    <none>              <none>              93407101859d        17 minutes ago      35.40 MB (virtual 171.77 MB)
    <none>              <none>              534f8b6157c4        17 minutes ago      56.7 kB (virtual 207.17 MB)
    <none>              <none>              5569fed43e24        15 minutes ago      355.28 MB (virtual 302.00 MB)
    <none>              <none>              d1d24098d412        17 minutes ago      92.12 MB (virtual 207.23 MB)
    <none>              <none>              c2aa9bb4cd55        14 minutes ago      12.0 kB (virtual 659.54 MB)
    <none>              <none>              0037757cab2b        17 minutes ago      2.65 MB (virtual 299.35 MB)


The size is computed in StoreImage (called while import or pull) and also in graph.Get for previously downloaded images.
The size of the parent is calculated every time in PS (it's very fast)


$>docker ps -a

    ID                  IMAGE               COMMAND                CREATED             STATUS              PORTS               SIZE
    2a151609665e        c2aa9bb4cd55                               Exit 127            15 minutes ago                          16.0 kB
    322a8f6c6b4e        712e29ecc84c                               Exit 127            15 minutes ago                          16.0 kB
    b85c456df6c1        5569fed43e24        /bin/sh -c wget http   Exit 0              15 minutes ago                          2.27 MB
    6f869614e4f9        0037757cab2b        /bin/sh -c apt-get i   Exit 0              18 minutes ago                          355.28 MB
    7f443142467e        d1d24098d412        /bin/sh -c echo orac   Exit 0              18 minutes ago                          2.65 MB
    647f7b708bcf        534f8b6157c4        /bin/sh -c apt-get u   Exit 0              18 minutes ago                          92.12 MB
    306a574de7be        93407101859d        /bin/sh -c apt-add-r   Exit 0              19 minutes ago                          56.7 kB
    8c0a903a4b78        base:ubuntu-12.10   /bin/sh -c apt-get i   Exit 0              20 minutes ago                          35.39 MB


The size if calculated each time.