Made an exec() program-chaining function for the Commodore libraries.
Added the model-specific addresses that the CBM exec() needs.
Added test programs for the exec() function.
Added a little more info about how CBM program-chaining works.
Merge pull request #32 from groessler/something_to_pull

minor improvements
Changed some CPU memory-mapped register definitions from expression mode into label mode.
Merge pull request #31 from greg-king5/cbm-chain

Commodore exec() function
em-test.c: add support for Atari