Adding snaktype property.

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

mwclient/entity.py

@@ -171,7 +171,10 @@ class Claim(object):
     Attributes:
         prop (str): property id.
         snak (dict): snak with all values return in mainsnak from API call.
+        snaktype (str): 'value', 'somevalue' or 'novalue'
         datatype (str): datatype ('wikibase-item', 'string', etc.)
+        value (dict): content of snak['datavalue']['value'] if snaktype is
+            'value', None othewise.
         string (str): string value, None if not 'string' datatype
         item (Item): Item value, None if not 'wikibase-item' datatype
         property_value (Property): Property, None if not 'wikibase-property'
@@ -181,12 +184,13 @@ 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 +201,14 @@ 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."""