Encode file_id as UTF-8 before hashing.

On Python 3.3.5 I get the following output when trying to compile anything with squiffy:

```python
> squiffy.py example.squiffy
Traceback (most recent call last):
  File "squiffy.py", line 355, in <module>
    process(sys.argv[1], os.path.abspath(os.path.dirname(sys.argv[0])))
  File "squiffy.py", line 15, in process
    story.set_id(os.path.abspath(input_filename))
  File "squiffy.py", line 306, in set_id
    self.id = hashlib.sha1(file_id).hexdigest()[0:10]
TypeError: Unicode-objects must be encoded before hashing
```

Encoding the file_id as utf-8 before hashing fixes it.