Add support for running nodes as part of the build process.

This should replace the "horrible hack" that we are currently
doing inside mesa build tarballs.

This code adds support for having the _result_ of a run be part of a
build.  See network_test.go for an example.

safety

safety
Merge pull request #33 from iansmith/allow_runs_in_build

Add support for running nodes as part of the build process.