Add missing layout functions

[oubiwann]

I've had to update svg.erl with an error message in the event that a layout tag is encountered, since the called function (svg_layout:add/3) doesn't exist (no such module).

I've had to comment out two tests that call a non-existent function svg:layout/2. I can only assume this, like the missing module/func above represented with that was in-progress and never pushed to Github.