Update badges to use "Shields" design + retina support

I modified the badges to use the popular [Shields](https://github.com/olivierlacan/shields) designs by @olivierlacan.

* I used the Travis badges as a starting-point,
* change the color of `pending` to a blue tone (as was already used by the original badges),
* used the original travis-yellow for the `unstable` Jenkins badge

I also removed the `Markdown` option, as it doesn't support retina images (no support for fixed-height images).