Added tablex.sort and tablex.sortv to iterate over sorted elements

There are times when it would be convenient to iterate over a table
either by sorted keys or values. There is an example of this in the
PIL book. I added the two functions to tablex module, added two test
cases, and updated the PL manual.
Merge pull request #53 from pkazmier/master

Added iterators to allow traversing tables in a sorted manner