Fix incorrect wrapping that fine-grained progress introduced

Sorry, my fine-grained progress commit somehow increased lengths and messed up the calculation with magic number in this line, resulting in this:

```
 marca@marca-mac2.lan ⮀ git-repos/pytest-sugar ⮀ ⭠ fix-wrapping s? ⮀ pytest-sugar ⮀
 ❯ py.test _test_supercalifragilisticexpialidocious_is_from_mary_poppins.py
Test session starts (platform: darwin, Python 2.7.6, pytest 2.5.2, pytest-sugar 0.3.0)
plugins: sugar

   _test_supercalifragilisticexpialidocious_is_from_mary_poppins.py ✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓5% ▉▉▉▉▉▉▉▉▌▉▉▍▉▉
                                                                    ✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓                                                            100% ▉▉▉▉▉▉▉▉▉▉▉

Results (0.78s):
   100 passed
```

With this PR:

```
 marca@marca-mac2.lan ⮀ git-repos/pytest-sugar ⮀ ⭠ fix-wrapping ? ⮀ pytest-sugar ⮀
 ❯ py.test _test_supercalifragilisticexpialidocious_is_from_mary_poppins.py
Test session starts (platform: darwin, Python 2.7.6, pytest 2.5.2, pytest-sugar 0.3.0)
plugins: sugar

   _test_supercalifragilisticexpialidocious_is_from_mary_poppins.py ✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓    85% ▉▉▉▉▉▉▉▉▌▉▉
                                                                    ✓✓✓✓✓✓✓✓✓✓✓✓✓✓✓                                                                         100% ▉▉▉▉▉▉▉▉▉▉▉

Results (0.77s):
   100 passed
```

I don't understand how this line works and I don't like the magic number. I tried pretty hard to replace it with something that uses `real_string_length(append_string)` but I couldn't find an offset that would achieve the right behavior. Maybe you can figure out how to do this calculation in a way that it doesn't get affected by changing the number of characters in the progress bar.