Add ps args to docker top

docker top works like before:

    $>docker top `docker ps -q`
    PID                 TTY                 TIME                CMD
    21417               pts/4               00:00:00            sleep

But now you can a use ps args:

    $>docker top `docker ps -q` a x u #or aux
    USER                PID                 %CPU                %MEM                VSZ                 RSS                 TTY                 STAT                START               TIME                COMMAND
    root                21417               0.2                 0.0                 4312                356                 pts/4               S+                  10:36               0:00                sleep               100

As this is a big change, and the top feature it's pretty new, it's not backward compatible (a warning is displayed)

Fixes #1237