This commit fixes serving of newly added files

This commit handles the issue in serve command which cannot host
new files. It is worth noting that new file content won't appear
until the tags are added.

Fixed #19
**Remark, not a rule so it shouldn't be changed without a reason**

You know, I think the following variant is a bit readable:

```
documents_paths = list(
    iterfiles(self.conf['paths.content'], '[!_.]*', True))
```

The idea is to *emphasize* that `self.conf['paths.content'], '[!_.]*', True` are arguments of `iterfiles`. That's my vision of such things, it shouldn't be changed since I'm ok with  your variant too. :)