Exported CSV's sometimes generate a broken filename (1)

It seems that the export of CSV files from Quizzard generates broken filenames. It seems like that sometimes an incomplete filename is generated, or filenames with unsupported characters tend to get generated.

The bug can be found in two places: export attendence and export results.

• Replace all non-alphanumeric (\w) characters in audience and quiz names with an underscore (1).