Makefile change: no more get-alpha

[Brian Rice]

I've updated the top-level Makefile to automatically call wget to  
retrieve newer VM's/images if they are not present in the main  
directory. This introduces an implicit wget and gmake dependency  
(which I would like to avoid), but means that new users should be  
able to get going more quickly.

If you have an existing setup, running "make full-clean" will do a  
"make clean" as well as wipe out your bootstrap files, resulting in a  
re-download. "make update-vm" will do a full-clean and then a build  
step.

These aren't great choices for terminology, but they're better than  
the "get-alpha" idea. :) Suggestions are welcome.