Fix app.width on OSX 10.7+

Addresses #625

After a bit of research it looks like this should be a pretty quick fix. After OSX 10.7, the scroll bars are included in the client area since they disappear when not scrolling. That means the current app#width gives an app width that is too wide by 16 pixels (scroll bar width) on OSX 10.7+.

I chose to have the os name and release number initialized because after experimentation, I learned that `uname` is a pretty slow command so we don't want to call it more times than necessary. 

I'm definitely not an expert on this, and I could only test it on my own machine: OSX 10.9., so please check this out if you're on a different OSX!