Add option to customize the output format of the "pip list" commad

As suggested in the issue #752 this pull request add a new option to pip list command which enable the possibility to customize the output format. An example of generation of a CSV output would be:

    $ echo "Name,Version" && pip list --format "{name},{version}"
    Name,Version
    git-remote-helpers,0.1.0
    wsgiref,0.1.2
    mock,1.0.1
    nose,1.2.1
    ScriptTest,1.2
    virtualenv,1.8.4

And a more elaborated example: 

    $ CONTENT=`pip list --format '{{"name":"{name}","version":"{version}"}},'` && echo "[${CONTENT%,}]"
    [{"name":"git-remote-helpers","version":"0.1.0"},
    {"name":"wsgiref","version":"0.1.2"},
    {"name":"mock","version":"1.0.1"},
    {"name":"nose","version":"1.2.1"},
    {"name":"ScriptTest","version":"1.2"},
    {"name":"virtualenv","version":"1.8.4"}]

Above is an example of generation of json like output using the new format option with a little effort. So, as you can see users can easily create customs outputs which can be much more easily parseable by automated tools.