Add unicode support

Now passing any unicode url raises a UnicodeEncodeError error:

``` python
>>> import furl
>>> url = u'http://ru.wikipedia.org/wiki/Достоевский,_Фёдор_Михайлович'
>>> furl.furl(url)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/shvechikov/Library/Python/2.7/lib/python/site-packages/furl/furl.py", line 805, in __init__
    self.load(str(url))  # Raises ValueError on invalid url.
UnicodeEncodeError: 'ascii' codec can't encode characters in position 29-39: ordinal not in range(128)
```

I've added automatic encoding unicode strings to utf-8.