Fix decimal separator issue

With my las commit, everythings was working fine in windows, but when I
try under Linux, I realized something :
Under Linux, ghostscript was sending back ink coverages values with
comma as separator, whereas it use point separator under windows.

It also explain the ClassCastException before my last commit -> the
Decimal formatter was expecting comme as separator.

So here is my solution.
Merge pull request #20 from BusyBusinessCat/master

Value parsing fix