Statusline: check vtd loaded OK
No global call to EnsurePythonLoaded

This can cause... problems, if the plugin did not load.  Instead, only
call EnsurePythonLoaded where we need the python.
Warn, don't load, if dateutil not installed

Fixes #8.

It's not perfect -- the message can get thrown multiple times if, e.g.,
the user has statusline set.  But it's way better than before.