Improved tables

Added the ability to define table colspans using empty pipe symbols.  If you want to span a column you just need to do the following:
<pre>
   | h1        | h2  |   h3 |
   |:----------|:---:|-----:|
   |c1         | c2  |  c3  |
   |c1         | c2         ||
   |c1         ||       c2  |
   |c1                      |||
</pre>
This change required some changes to the internal structure for the child nodes.  I modified the needed tests and the latex generator to handle this internal change.