onExit() is similar to the onLoad() callback. onExit() is called on each
module just before the process exits. This can be used to check state in
unit tests, but not to perform I/O. The process will forcibly exit as soon
as all of the onExit callbacks are made.
There is one major API change in the refactor: filename extensions are now
required when requiring or including modules.
Added extra test to test-module-loading.js.
All the c++ code is now reduced to simple wrappers. The node.fs.File object
is defined entirely in javascript now. As is the actionQueue methods.
This makes the boundaries much cleaner. There is still some thought that
needs to go into how exactly the API should behave but this simplification
is a first step.
