Implement longtable support for LaTeX export.
Exporting tables to LaTeX suffered from the problem that Org tables
are often long, but that the tabular environment in LaTeX cannot
handle long tables. This patch fixes this issue.
LaTeX export can now deal with very long tables, by setting an option
in the "ATTR_LaTeX:" line. Also, you can specify the alignment by
hand. Here is an example:
Also, table export now correctly deals with tables that start with a
hline. In such a case, also the LaTeX version will have this extra
hline before the first line.