Use translitcodec for urlify

[```urlify```](https://github.com/web2py/web2py/blob/231fa489cbe35408dba0ee41253750a065fa1fff/gluon/validators.py#L2505) does its job and does it well. However, for some languages ```unicodedata.normalize('NFKD', s)``` does not lead to a common replacement. For example
https://de.wikipedia.org/wiki/Gr%C3%BCn
is rather converted into
https://de.wikipedia.org/wiki/Gruen
than into
https://de.wikipedia.org/wiki/Grun.

[translitcodec](https://pypi.python.org/pypi/translitcodec) has a built-in translation table for common replacements which makes use of Markus Kuhn’s [transtab package](http://www.cl.cam.ac.uk/~mgk25/download/transtab.tar.gz).