[Images / MarketPlace] : add a list and a show function

I wrote two little functions.

It requires mechanize python package.

The first one: "stratus-list-images" ask the marketplace what images are avalaible by parsing the metadata returned by the endpoint URL.
The second one: "stratus-show-image" need a VM ID as an argument:
stratus-show-image -i $VM_ID
It lists every informations on this VM.

We should also code "search" function (by author / os...) using the same python packages.

Maybe it needs to be better integrated with the functions included in StratusLab.

I hope it could help.

Regards,
Remy Dernat.