Add support for High Resolution displays

This adds support for High Resolution displays.

Tested and works perfectly on OS X Safari, Chrome and Firefox
Not tested on Non-Retina screen, but I don't see why it would change anything on those.

The amount of changes to the code is very small, and performance impact is pretty much inexistent.
This adds an option called "PixelRatio" to graphs. This option is set automatically on initialization. Maybe it should not be an option but a property of the Network object.

Important note : 
Rendering of nodes and edges is impacted in some way on Retina Screens.
For example, if edge width is set to 1px, it will be rendered with 1px on non retina screen, and 1px on retina screens, which will make it look half the width. 
Modifying this behavior requires a bit of further work.