String::dirify converts a string (possibly containing high ascii characters, and even html) into another, lower-cased, string which can be used as a directory name or url.

the algorithm is as follows:
* each high ascii character is replaced by its normal equivalent
* the string is converted to lower case
* any html (including html entities) in the string is removed
* any characters which are not (perl) words, spaces or hyphens, are removed
* runs of spaces are converted to the separator character