Use one shutdown_event and one configuration reloading thread

Some notes:

- `shutdown_event` object and `ConfigLoader` instance must be created outside of `__init__` and passed as an argument. Otherwise there will be one ConfigLoader and one shutdown_event per Powerline; and threads will use whatever shutdown event was the last one to come.
- `shutdown` is no longer supposed to be called in any case except for just before shutdown.

Also adds `interval` configuration, forbids querying vim from non-main threads (is not really useful in the current state though), takes tests for `inotify` watcher and adds a note about vim-addon-manager.