Attribute error: 'Spreadsheet' object has no attribute 'clean_pointers'

Hello,

The libray is working well but when I try to clean pointers I have an error:

```
from koala.Spreadsheet import Spreadsheet
from koala.ExcelCompiler import ExcelCompiler
sp = Spreadsheet(file)
sp.clean_pointers()
```

It says : Attribute error: 'Spreadsheet' object has no attribute 'clean_pointers'