#3 #4 Internal directory structure, additional commands implemented.

Implemented the following commands:
* cd - to change directories
* echoErr - just like "echo" but to STDERR

Also, internal directory state is now tracked. This allows things like
dynamically changing the the PROMPT text to the current directory,
renaming the tab name (coming in #10), and changing directories.

There is one more change for internal directory state: make sure
ProcessBuilder knows to use the current working directory when starting
a command.