New: possibility to draw automatically in high DPI in canvas2D

Previously, Cappuccino didn't handle retina device when drawing for canvas2D. Now it does.
To do that, Cappuccino will firstly calculate the pixel ratio of the current device, then it needs to change the css style of the canvas by multiply it by the current pixel ratio and finally scale the canvas by this pixel ratio.

More information about high DPI drawing here : http://www.html5rocks.com/en/tutorials/canvas/hidpi/

Added the method `setAllowsHighDPIDrawing:` and `allowsHighDPIDrawing` to deactivate or activate this feature.

Fixed #2175
Don't repeat the return value description.

```
Returns whether high DPI drawing is enabled.
@return BOOL - YES if high DPI drawing is activated, otherwise NO.
```