Use integer divison on two lines to fix TypeErrors

Fixes this error:

```
 marca@marca-mac2.corp.surveymonkey.com ⮀ git-repos/python-texttable ⮀ ⭠ py3 ? ⮀
 ❯ python3.3 example.py
Traceback (most recent call last):
  File "example.py", line 12, in <module>
    print(table.draw() + "\n")
  File "/Users/marca/dev/git-repos/python-texttable/texttable.py", line 359, in draw
    out += self._draw_line(row)
  File "/Users/marca/dev/git-repos/python-texttable/texttable.py", line 586, in _draw_line
    line = self._splitit(line, isheader)
  File "/Users/marca/dev/git-repos/python-texttable/texttable.py", line 669, in _splitit
    cell[:0] = [""] * (missing / 2)
TypeError: can't multiply sequence by non-int of type 'float'
```