Fix bug with non ascii passwords in notebook login

[lev112]

When a password is entered with non ascii characters, a 500: Internal Server Error is returned.
I don't think non ascii passwords should be supported, but a 'Invalid password' message should be returned.

2013-11-12 11:43:19.608 [tornado.application] ERROR | Uncaught exception POST /login?next=%2F (127.0.0.1)
HTTPRequest(protocol='http', host='127.0.0.1:8888', method='POST', uri='/login?next=%2F', version='HTTP/1.1', remote_ip='127.0.0.1', headers={'Origin': 'http://127.0.0.1:8888', 'Content-Length': '63', 'Accept-Language': 'en-US,en;q=0.8', 'Accept-Encoding': 'gzip,deflate,sdch', 'Host': '127.0.0.1:8888', 'Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8', 'User-Agent': 'Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/30.0.1599.101 Safari/537.36', 'Connection': 'keep-alive', 'Referer': 'http://127.0.0.1:8888/login', 'Content-Type': 'application/x-www-form-urlencoded'})
Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\tornado\web.py", line 1141, in _when_complete
    callback()
  File "C:\Python27\lib\site-packages\tornado\web.py", line 1162, in _execute_method
    self._when_complete(method(*self.path_args, **self.path_kwargs),
  File "C:\Python27\lib\site-packages\IPython\html\auth\login.py", line 48, in post
    if passwd_check(self.password, pwd):
  File "C:\Python27\lib\site-packages\IPython\lib\security.py", line 116, in passwd_check
    h.update(cast_bytes(passphrase, 'utf-8') + str_to_bytes(salt, 'ascii'))
UnicodeDecodeError: 'ascii' codec can't decode byte 0xd7 in position 0: ordinal not in range(128)

[takluyver]

I think we should aim to support non-ascii passwords properly. I'll look into it.

[takluyver]

Should be fixed in PR #4526.