Fix width calculation for individual line numbers

Since the gutter wrapper for each individual line number element has a `left` of `-totalGutterWidth`:

![Gutter wrapper left](http://f.cl.ly/items/2Y3G3T2c190D2t1o3B14/Image%202014-10-04%20at%203.50.02%20PM.png)

 and the line number element needs to take up the whole width in the gutter wrapper, it should be `gutterTotalWidth - padding`, not `lineNumInnerWidth`.

Otherwise, if the `CodeMirror-gutters` element has borders or such (for instance, separating the gutter from the text), the line number element will be misaligned within its wrapper: 

![Misalignment](http://f.cl.ly/items/390I3P0p2v0P0S2a2z16/Image%202014-10-04%20at%203.52.32%20PM.png)

This commit should fix the issue:

![Fixed](http://f.cl.ly/items/3R1z2k0o3e3G3V18072w/Image%202014-10-04%20at%203.57.30%20PM.png)