Add .delete() to all Storages.

Reviewed in http://codereview.appspot.com/5608049/.
diff --git a/oauth2client/appengine.py b/oauth2client/appengine.py
index e4169e9..f97ce69 100644
--- a/oauth2client/appengine.py
+++ b/oauth2client/appengine.py
@@ -263,6 +263,16 @@
     if self._cache:
       self._cache.set(self._key_name, credentials.to_json())
 
+  def locked_delete(self):
+    """Delete Credential from datastore."""
+
+    if self._cache:
+      self._cache.delete(self._key_name)
+
+    entity = self._model.get_by_key_name(self._key_name)
+    if entity is not None:
+      entity.delete()
+
 
 class CredentialsModel(db.Model):
   """Storage for OAuth 2.0 Credentials
diff --git a/oauth2client/client.py b/oauth2client/client.py
index 2d60f5a..f492563 100644
--- a/oauth2client/client.py
+++ b/oauth2client/client.py
@@ -224,6 +224,13 @@
     """
     _abstract()
 
+  def locked_delete(self):
+    """Delete a credential.
+
+    The Storage lock must be held when this is called.
+    """
+    _abstract()
+
   def get(self):
     """Retrieve credential.
 
@@ -252,6 +259,21 @@
     finally:
       self.release_lock()
 
+  def delete(self):
+    """Delete credential.
+
+    Frees any resources associated with storing the credential.
+    The Storage lock must *not* be held when this is called.
+
+    Returns:
+      None
+    """
+    self.acquire_lock()
+    try:
+      return self.locked_delete()
+    finally:
+      self.release_lock()
+
 
 class OAuth2Credentials(Credentials):
   """Credentials object for OAuth 2.0.
diff --git a/oauth2client/file.py b/oauth2client/file.py
index d71e888..1abc6d2 100644
--- a/oauth2client/file.py
+++ b/oauth2client/file.py
@@ -85,7 +85,6 @@
       finally:
         os.umask(old_umask)
 
-
   def locked_put(self, credentials):
     """Write Credentials to file.
 
@@ -97,3 +96,12 @@
     f = open(self._filename, 'wb')
     f.write(credentials.to_json())
     f.close()
+
+  def locked_delete(self):
+    """Delete Credentials file.
+
+    Args:
+      credentials: Credentials, the credentials to store.
+    """
+
+    os.unlink(self._filename)
diff --git a/oauth2client/multistore_file.py b/oauth2client/multistore_file.py
index d9b89c8..1f756c7 100644
--- a/oauth2client/multistore_file.py
+++ b/oauth2client/multistore_file.py
@@ -159,6 +159,17 @@
       """
       self._multistore._update_credential(credentials, self._scope)
 
+    def locked_delete(self):
+      """Delete a credential.
+
+      The Storage lock must be held when this is called.
+
+      Args:
+        credentials: Credentials, the credentials to store.
+      """
+      self._multistore._delete_credential(self._client_id, self._user_agent,
+          self._scope)
+
   def _create_file_if_needed(self):
     """Create an empty file if necessary.
 
@@ -344,6 +355,23 @@
     self._data[key] = cred
     self._write()
 
+  def _delete_credential(self, client_id, user_agent, scope):
+    """Delete a credential and write the multistore.
+
+    This must be called when the multistore is locked.
+
+    Args:
+      client_id: The client_id for the credential
+      user_agent: The user agent for the credential
+      scope: The scope(s) that this credential covers
+    """
+    key = (client_id, user_agent, scope)
+    try:
+      del self._data[key]
+    except KeyError:
+      pass
+    self._write()
+
   def _get_storage(self, client_id, user_agent, scope):
     """Get a Storage object to get/set a credential.