Barrier taks queue
Print the barrier taks queue.
Catch invalid object ID.
Add subcommand semaphore
rtems semaphore <index(s)> : prints rtems semaphores by index number
Add task subcommand
rtems tasks <index(s)> - Prints tasks by index.
Catch nonvalid indexes.

Catch IndexErrors generated while referancing non existant indexes.
Add message_queue subcommand.
Refactor

 - pretty printers moved to pretty module
 - command and subcommands get own module
Categories the commands.

The subcommands fall onto DATA.
See http://sourceware.org/gdb/onlinedocs/gdb/Commands-In-Python.html#Commands-In-Python
Refactor subcommands
 - index commands inherit from a parent class.