Fix #34: Add node to path for NodeExec.

 - Prepend the node bin directory to the `PATH` in the NodeExecRunner
   for downloaded / managed node installations.

Thanks to @time4tea for the easy reproduction steps and solution code. It was just a matter of putting it in the right place. Let me know if there is a better place to inject the node bin directory in the `PATH`. I am going to try my hand at writing some tests for this in the morning - I should have done that to start with.