This greatly enhances the code for p4-trigger-script, bringing in some

functionality that will be available in the upcoming rbt repo-hook

command for RBTools 5.
The script now allows for command line options for customizing behavior

and connection information, which means that users don't have to fork

and modify the script.
It also allows admins to specify certain arguments on the command line

that will be expanded when calling the trigger. This is needed to

talk to Perforce and edit changes as the submitting user.
New in-script settings (and command line options) are added for enabling

debugging, setting a p4trust file location, setting a Perforce client

name and host, and setting a Review Board API token.
Documentation is provided at the top of the script to help admins get

set up. Docs on the website will be created separately, once this is

fully tested in production.
Some changes had to be made within RBTools to facilitate some of these

changes:


initialize_logging() now accepts a debug parameter to enable

   debug logging for hooks.


The call to p4 change no longer ignores error result (which caused

   errors to be sent to the caller as a valid changeset).


Logging information for failed marshalled commands now includes

   demarshalled payload and stderr information.

Thoroughly tested the script with command line options and in-script

settings.
Tested with using a hard-coded user and a Perforce-provided %client%,

%clienthost%, and %user%.
Tested with and without debugging information.
Tested with inaccessible Perforce changesets (which unveiled the

p4 change error handling problem).