Add support for SUBSTITUTE

[todhunter]

Could be great with support for the SUBSTITUTE function.
I was able to get the substitute to work by adding the substitute function to koala/excellib.py

def substitute(*args):
    if tuple(flatten(args)) != args:
        return ExcelError('#VALUE', 'Could not process arguments %s' % (args))

    cat_string = str(args[0]).replace(str(args[1]), str(args[2]))

    return cat_string