minor whitespace cleanup, fixes #87

I'm not familiar with auto-layout, but I attempted some whitespace tweaks nonetheless. Hopefully I didn't botch it too badly. Basically I did two things:

- set whitespace between label and field to 6
- set whitespace below field to 12

XCode made some random changes to the XIB, unfortunately. I used 4.6.3 if that helps. Not sure what version you're on.