Parallel pull

Download images from a repo in parallel, `tianon/debian` is a good exemple:

    $> docker pull tianon/debian
    Pulling repository tianon/debian
    01468af711dc: Downloading 44.89 MB/167 MB (27%)
    08c9d0ea8240: Downloading 39.08 MB/162 MB (24%)
    764a25351209: Downloading 60.07 MB/60.07 MB (100%)
    90453b4737d4: Downloading 66.54 MB/165.1 MB (40%)
    a1390ca6935c: Downloading 62.85 MB/162 MB (39%)

I'm waiting #1238 to be merged to use parallel pull in API version 1.4 and keep the old way in 1.3 
Related to #150