fix progressbar in docker push

Fixes #3260

Changes:

+ The registry code will now try to close the "layer" after a successful `PUT`
+ The API will send a JSON says 100% on "layer" close

In addition to have a nicer display, 

* ProgressBar code cleaned up a little (removed commented code and useless `io.ReadCloser` cast)
* Server send `Image successfully pushed` 
* Don't display time left when it's 100%