partial work: in the plugin manager, if you click a plugin that is already loaded then unload it.  todo: remove it from the menu, do not allow this action if the plugin is used in the notebook.
detect if user tries to delete plugin that is being used, display warning box instead
remove the evaluator menu items for the plugin when removing the plugin
remove debug logging
remove more debug logging
Merge pull request #575 from twosigma/spot/77

allow removing plugins from notebook, plus other bug fixes