model exec to add the ability to run multiple different binaries

[GoogleCodeExporter]

Reported by Cristian Zamfir.

Currently Cloud9 only allows a single binary to be loaded and models fork() to 
provide multi-processor support and test distributed systems. Ideally, exec 
should also be supported and allow Cloud9 to load different binaries. This 
would be a better way to provide support for testing distributed systems

Original issue reported on code.google.com by stefan.b...@gmail.com on 31 Aug 2012 at 4:01