Extract can methods

This is a minor refactoring that possibly helps to improve the readabiity of the mapping files to resources.

Actually, I am still a bit confused about what mappings are possible:

* single file --> single resource (any file or specific file)
* directory --> index resource (default path) and taken care of by `_canDirectory`

If this is correct, maybe it would help to extract one more method: `_canSingleFile` or similar. Then, the main mapping function would become:

<pre>
 fs.stat(httpObj.path + '/' + httpObj.dname, function (err, stats) {
      if (err) {  // file exsits
          that._canSingleFile(httpObj);
        })
      } else {
        if (stats.isDirectory()) {
          that._canDirectory(httpObj);
        } else {
          new Response('html', '', 500, httpObj.res).send();
        }
      }
</pre>

And it would be clearer to me how it works. What do you think?