Add documentation for 'Table Totals'

Document change in #178 

 - also need to add grandTotal strategy, where Table would perform the query.


``` php
$t->addTotals(); // todo - must specify strategy.
```

Also need to create hooks for outputting columns before and after the main body of the table, where we can.

Need the ability to create groups in a table with headers and footers (such as sub-totals).