Better error message for upper-case() w/o argument

When called without an argument, we raise a warning.
More translations

...when will they ever end? When will they ever end?
New option for <NewPage> to skip pages
Fix source code documentation with locco

Update markdown to use with Lua 5.2 and make some fixes
to locco to properly escape % in non-strings.
More locco update to make it html5 conform
QA/pagediff adopt to new version of imagemagick

The return code of compare seems to be now (how was it before?)
0 no difference in the given images
1 if the comparison is OK, but with differences
2 if there is a fatal error
Bugfix: default main language

This slipped through the commit 927bbcdb7...
Source code documentation

Add images to `tabular.lua' and add mathjax for formulae
Some more documentation for tabular.lua
Template for mathjax in sourcedoc
Schema / documentation for collapse
Border-collapse for horizontal cells

This is the easy case: no problems with splitting tables
Vertical border-collapse

Note: cases where rowspan > 1 will be handled later on.
* bordercollapse:
  Vertical border-collapse
  Border-collapse for horizontal cells
  Schema / documentation for collapse
Default h-alignment of td now "left"

Due to a global variable, the default alignment of the table cell was
somewhat non-deterministic.
Bugfix: tabular row height calculation

The default text format differed from the calculation and the real
typesetting.
Bugfix tabular horizontal alignment

The horizontal alignment should default to »left« for each cell. 
I didn't clear the variable between each cell in a row, thus an
alignment in a cell affects the alignment in the next cell.
Bugfix tabular (no) border collapse

When border-collapse is not used, the shift-up can be nil. A 
comparison later on (nil > 0) makes the program crash.
New horizontal alignment for table cells: justify

The default is now _left_, _justify_ gets the old behavior back.
Well, sort of. The old behavior was totally broken so it was
impossible to rely on a default alignment in a table cell. So in
practice, this should not affect any existing layout.
Schema enhancement (small change)