adding snaktype property.

Claim object have now both properties:
* `snaktype`
* `value` set to `None` if `snaktype` is either `somevalue` or `novalue`

mwclient/entity.py

@@ -181,12 +181,13 @@ class Claim(object):
         quantity (dict): quantity dictionnary, None if not 'quantity' datatype
     """
 
-    def __init__(self, site, prop, datatype, value, snak=None):
+    def __init__(self, site, prop, datatype, snaktype, value=None, snak=None):
         """Constructor"""
         self.site = site
         self.prop = prop
         self.datatype = datatype
         self.value = value
+        self.snaktype = snaktype
         self.snak = snak
 
     @classmethod
@@ -197,9 +198,14 @@ def fromsnak(cls, site, snak):
             site (mwclient.WikiBaseSite): site
             snak (dict): snak dictionnary
         """
+        snakvalue = None
+        if snak['snaktype'] == 'value':
+            snakvalue = snak['datavalue']['value']
         return cls(site, snak['property'],
                    snak['datatype'],
-                   snak['datavalue']['value'])
+                   snak['snaktype'],
+                   value=snakvalue,
+                   snak=snak)
 
     def __repr__(self):
         """Representation."""