I added a little verbage to hashtab for the hashtab_destroy func.

I added a little verbage to hashtab for the hashtab_destroy func.
It was pretty sparsely documented.

This update fleshes out the pbx_lua module, to
add the switch statements to the extensions in the
extensions.lua file, as well as removing them when
the module is unloaded.

Many thanks to Matt Nicholson for his fine
contribution!

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@144523 65c4cc65-6c06-0410-ace0-fbb531ad65f3