Support for high-DPI displays

Currently gauge.js looks rather blurry on high-DPI displays such as retina macbooks, or the new kirabook from toshiba. This pull request uses the technique described at [html5rocks](http://www.html5rocks.com/en/tutorials/canvas/hidpi/) to achieve very sharp gauges on these displays as well.

*  [Before](https://f.cloud.github.com/assets/1258798/490110/872b5f0e-b9b0-11e2-8a2b-52f330dea20b.png)
*  [After](https://f.cloud.github.com/assets/1258798/490111/8c88ed2c-b9b0-11e2-94c6-4c4ecb86a4f9.png)

Note I did not commit the recompiled js-file as I was unsure if you would want me to. Let me know if I should add it.