Support for running images

[butla]

Hey! I think the idea behind this plugin is great, but I'm missing a support for running images. Something like that:

def spawnu_im(repository, tag=None, image_id=None, options=None):
    # either a tag or an image_id needs to be provided
    # pulls an image if it's missing; this should minimize test setup
    # run the image with the given options (e.g. ports)

Do you have any ideas on that? If I would create a pull request, would you be willing to merge it?