Podex handling multiple images

Podex can now handle multiple images.
Also adding `podID` flag, with which user can specify the pod ID.
eg:
   `podex -json -podID=nodejs dockerfile/redis google/nodejs-hello`

In case the command will take only one image the `podID` is optional and image base-name will be used for as a podID 
and godoc, rename to parseDockerImage