Fix #288 - fs.unlink should fail on directory nodes

Didn't we find that node threw an `EPERM`?  From the [unlink spec, Errors section](http://pubs.opengroup.org/onlinepubs/009695399/functions/unlink.html):

<blockquote>
[EPERM]
    The file named by path is a directory, and either the calling process does not have appropriate privileges, or the implementation prohibits using unlink() on directories.
</blockquote>

We *might* have EPERM commented out in src/errors.js (i.e., previously unused), FYI.