exhibit serve should watch for changes

[moggers87]

Either we can be smart and check things like content_path and templates (though we'd have to set that up on a per Node basis as it can be changed arbitrarily) or we can just watch the CWD for changes.

[moggers87]

Django does this: https://github.com/django/django/blob/master/django/utils/autoreload.py#L342

Might be useful for detecting if a node has changed