Bjorne progress

I've made significant progress on the bjorne shell. It can tokenize and parse most of the grammar (apart "here" documents), and it can execute shell variable assignments and simple commands with simple '$' expansions.

I'm now running into a number of issues in the nexus between the shell and invoked commands. These include how to support return codes, numbered file descriptors, PIDs/job control, environment variables and the current directory. I'll talk about these in future blog entries.