Update mandelbrot.py

Fixed a float to int bug. Windows7, Python 3.3, PySide 1.1.2

Traceback (most recent call last):
  File "mandelbrot3.py", line 102, in run
    for y in range(-halfHeight, halfHeight):
TypeError: 'float' object cannot be interpreted as an integer
Merge pull request #10 from Lazik/patch-1

Update mandelbrot.py