Adds notifications/dismiss/:id (Fixes #265)

@Kimik0 noticed the endpoint was missing. Read more about it here #265

Using-the-API/API.md

@@ -373,6 +373,13 @@ Returns the [Notification](#notification).
 Deletes all notifications from the Mastodon server for the authenticated user.
 Returns an empty object.
 
+#### Dismissing a single notification:
+
+    POST /api/v1/notifications/dismiss/:id
+
+Deletes a single notification from the Mastodon server for the authenticated user.
+Returns an empty object.
+
 ### Reports
 
 #### Fetching a user's reports: