Make tags and cats case insensitive. Fixes #704.

More precisely, group tags or categories without considering the case.
This fixes the bug where two categories with just the case as difference were
considered as distinct, but generate the same file: one overwriting the other.

Thanks to @Avaris for helping with the tests.

pelican/urlwrappers.py

@@ -13,21 +13,35 @@ logger = logging.getLogger(__name__)
 @functools.total_ordering
 class URLWrapper(object):
     def __init__(self, name, settings):
+        # next 2 lines are redundant with the setter of the name property
+        # but are here for clarity
+        self._name = name
+        self.slug = slugify(name)
         self.name = name
-        self.slug = slugify(self.name)
         self.settings = settings
 
+    @property
+    def name(self):
+        return self._name
+
+    @name.setter
+    def name(self, name):
+        self._name = name
+        self.slug = slugify(name)
+
     def as_dict(self):
-        return self.__dict__
+        d = self.__dict__
+        d['name'] = self.name
+        return d
 
     def __hash__(self):
-        return hash(self.name)
+        return hash(self.slug)
 
     def _key(self):
-        return self.name
+        return self.slug
 
     def _normalize_key(self, key):
-        return six.text_type(key)
+        return six.text_type(slugify(key))
 
     def __eq__(self, other):
         return self._key() == self._normalize_key(other)