Fix for zippyshare checksums

Update ZippyshareCom.py such that it matches the way that Zippyshare currently generate checksums (a*b+19)
I would replace with:
```python
m = re.search(r'<script type="text/javascript">[^<]*var b = (\d*)%(\d*);', self.html)
if m:
    b_value = m.groups()
else:
    self.parseError("Unable to extract B values")
```