[Plugins] Plugin types and deferreds

[gdude2002]

This adds the following to Ultros:

• Support for plugins of different types
  • This is achieved by allowing plugins to themselves register loaders for other plugin types
  • Naturally, this means that Python plugins have their own modular loader - this can be unloaded by plugins too, but I don't really recommend that
• Support for Deferreds in various places
  • The plugin manager (and factory manager) is now able to deal with plugins returning Deferreds in their setup() and deactivate() functions
  • Of course, this means that more parts of the plugin manager return Deferreds now - Specifically:
  • remove_loader(name)
  • load_plugins(plugins, output=True)
  • load_plugin(name)
  • unload_plugins(output=True)
  • unload_plugin(name)
  • reload_plugin(name)
  • The docstrings for these have been updated appropriately for the api docs