Adds sections on command registration and chaining, giving an overview
to developers that want to use these features.

Improve the developer manual and primer sections which talk about
writing command handlers.  Notably, it documents the new CMD_* macros.

Update the style guide and chase obvious references to structures
that have been renamed.

This documentation update provides an introduction to the command
handling facilities provided by command.[ch].  A primer walks the user
through the elements of a pointedly pedantic module: src/hello.c.

A summary of the API is provided in the OpenOCD Architecture section.

git-svn-id: svn://svn.berlios.de/openocd/trunk@1771 b42882b7-edfa-0310-969c-e2dbd0fdcd60