Scale the splash screen logo based on the size of the screen

[samreid]

Everything in a PhET simulation scales to fit the window except the splash screen logo/text. Should this scale as well? Tagging @ariel-phet @pixelzoom for discussion.

[ariel-phet]

I suppose that would be nice, but seems extremely low priority unless it would make the code cleaner/more consistent.

[pixelzoom]

I wouldn't bother with this, unlikely that the user will resize the browser window while the sim is loading, and if they do, not a big deal because it doesn't compromise functionality of the sim. Consider addressing this if/when animation is added to the splash, in #68.

[samreid]

I suspect this issue will become more important if/when we start targeting smaller devices (where the logo currently fills the whole screen or even goes offscreen).

[pixelzoom]

Folded this issue into #309, where the splash screen is getting a redesign.