Add documentation for timing modules used in Asterisk

This document specifies the timing modules available in Asterisk beginning
with Asterisk 1.6.1. The document goes into detail about the differences
between each and gives a general overview of what timing is used for in
Asterisk. There is also a section which can be used to help customize
your setup or to troubleshoot timing issues you may have.

I also added messages to the DAHDI timing test used in res_timing_dahdi.c
that points to this new documentation if people experience problems.

Big thanks to all who contributed comments on this.

(closes issue #14490)
Reported by: mmichelson
Patches:
      timing.txt uploaded by mmichelson (license 60)

Review: http://reviewboard.digium.com/r/164/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@179937 65c4cc65-6c06-0410-ace0-fbb531ad65f3