Make get_unverified_claims() return a dict

So far the above method returned the unprocessed string it received from
`_load()`.

This commit parses the payload before returning it in order to comply
with the documentation.

jose/jws.py

@@ -124,7 +124,11 @@ def get_unverified_claims(token):
  JWSError: If there is an exception decoding the token.
  """
  header, claims, signing_input, signature = _load(token)
- return claims
+
+ try:
+ return json.loads(claims.decode('utf-8'))
+ except ValueError as e:
+ raise JWSError('Invalid claims string: %s' % e)
 
 
 def _encode_header(algorithm, additional_headers=None):