add tests and documentation for itk.PyBuffer

```
It seems that the Windows DeVIDE 12.2.7 doesn't include PyBuffer, but the Linux 
version does.

Write tests for PyBuffer to exercise this.

Example use:
{{{
import itk
my_numpy_array = itk.PyBuffer[itk.Image.F3].GetArrayFromImage(your_itk_img)
}}}
```

Original issue reported on code.google.com by `cpbotha` on 23 Apr 2012 at 7:30
