Updated the Rakefile and mplab.rb to be aware of the default MPLAB X directory on Mac OS X.
In Mac OS X, the JAR files are stored in a different directory relative to the MPLAB X directory.  Added Mplab.jar_dir to account for that.  All the specs are passing on MAC OS X now.
Put the code for loading MPLAB X into its own class.  This makes it easier to understand and easier for users to override in case they have special needs.
mplab.rb: A little more cleaning up related to the constant shortcuts.
docs: Updated it to reflect that RPicSim has been tested on Linux and Mac OS X now.
Improved high-level comments for the RPicSim::Mplab classes.  Also put MplabObserver into the correct module.
Moved some requires from mplab.rb to mplab_loder.rb because that is where they are really needed.
Made 'rake todo' show TODO items that are in the Rakefile itself.
Fixed the problem with the way we were treating the stack on enhanced midrange devices.  Introduced a new class, StackPointer, and a method Sim#stack_pointer to get an instance of it.