Standardise module load order for native and registered file extensions.

This patch standardises the load order for modules. Highest priority is trying to load exactly the file the user specified, followed by native extensions, followed by registered extra extensions, etc.

In full, if we require('foo') having registered '.coffee' as an alternative extension, we try and load the following files in order:

    foo
    foo.js
    foo.node
    foo.coffee
    foo/index.js
    foo/index.node
    foo/index.coffee