Centering the screen is slightly wrong when there are long lines

[nullromo]

To reproduce

1. Create a file that has a really long line in it, or resize the window such that the line takes up 10-12 lines.
2. Put the cursor just below the long overflowing line.
3. zz, and notice that the current line is not centered.
4. :normal! zz<CR> and notice that it behaves differently from zz.

[nullromo]

This is likely related to #13. I imagine fixing one will also fix the other.