Improve code for better minification

[mgreter]

Thanks for the function, going to need it for responsive images.
Since I need to include this in the html head, I thought after
reviewing it, that it could be easily compressed even more.

I slighly adjusted and simplified the implementation a bit
to improve minification. Mainly by getting some base objects
like screen into variables, so the minfier only sees them once.

I also changed the checks a little, which should IMO be safe. You
explicitly check for !== undefiend, but I guess only positive numbers
really make sense in the later calculation. Therefore a simple || 1 seems
even a bit safer, as it protects from a possible division by zero.

Google closure compiler minfies the new code to 151 bytes, while
the current implementation is at 281 bytes 😃 I double checked that
I did not include any errors and tried to make sure the logic is sane.
Ie. it is now guaranteed to never return zero, which seems desirable!

Thanks and kind regards!