Misc. Test fixes and code cleanup.

`script.render.main()` now takes arguments which means that it's no longer
necessary to mess with `sys.argv[]` - the old tests were broken if you fed
arguments into nosetests.

It also improves naming of the line-segments which I wrote myself and found
confusing. I think it's better to call them "x-segments" (linesegments over
the x axis) rather than "horizontal segments".

I also put something in to blow up if there are no glyphs inside the
table bounding box and I want to know if that ever happens (it shouldn't
for now at least, maybe later we can make it continue rather than blowing
up)