4 bit fold

Hi Scott,

I've modified 'fold_psrfits' to work with 8-bit data. I've tested it with 4-bit data. I think the changes are fine based on a few tests, but I wasn't particularly thorough. The resulting profile looks reasonable when compared with that of prepfold. I also compared folds of 8-bit data using my modified version and the previous version. The binary data are slightly different, but the output plots look identical. 

Patrick