Fix an integer arithmetic bug, and improve the confidence multiplier.

This was previously PR dcramer/chardet#5, which fixes issue dcramer/chardet#3.

As reported there by @ablegrape:

>Following up on our exchange from a few weeks ago. I've commited the bug fix for the integer math bug (causes a file with even one "low confidence" character to have 0 confidence level) and also updated the confidence multiplier for latin 1 to one that works better (the original multiplier causes many files to be incorrectly detected as iso-latin-2). Testing on both the problem case mentioned ( http://www.lvo.com/GASTRONOMIE/VINS/VITI/VITI1F.HTML ) and the character set tables from http://www.columbia.edu/kermit/csettables.html suggests that the fixed code performs better.

This seems like a very helpful fix to merge.