Create Manager

For now, just some special class to make the HabiticaApi class shorter
and move API requests by type to inner classes, so you can write
api.Tags.List() and not api.ListTags()

Managers/Manager.cs

@@ -0,0 +1,7 @@
+
+namespace HabitSharp.Managers {
+    public abstract class Manager {
+        protected readonly HabiticaApi Client;
+        public Manager(HabiticaApi client) => this.Client = client;
+    }
+}