Update to the Z50Bus specification

[Jon Langseth]

As a result of talks I've had with Steve Cousins, and questions I have received elsewhere, it was about time that I made some small updates to the Z50Bus specification.

I've also made sure the URL http://linc.no/go/z50bus leads to the Z50Bus specification (it used to work, but that was pure luck, now it's locked to always point to the correct location, and will keep doing that even if I move the actual URL for the specification elsewhere).

The changes are:

 * Added recommendation of letting USR-pins be User-defined

 * Added recommendation of including jumpers/links on the RX/TX lines (to avoid issues on multi-UART systems)

 * Added nomenclature regarding Front/Back edges of cards

 * Added mention of connector placements, as per the question/answer on https://groups.google.com/forum/#!topic/linc80/PD2Nnwf50c0

 * Changed the naming of suggested/recommended card sizes

 * Updated the image showing physical dimensions and nomenclature

 * Changed the recommendation of Host-to-bus connectors slightly

 * Started a list of Z50Bus products

The list of products is currently just five lines long, but I hope that changes over time :)