load and remove now returns the result of the op
cosmetic change
added supervisor for certain plugins
gen_server plugins are possible

Some kind of plugins (i.e. control the bot, restart plugins) needs a
more powerful model, so it's possible to create them as gen_server. they
have a separate supervisor.
forgot

(should be in previous commit)
couple of cosmetic changes
refactored plugin_mgr

removed useless code, reorganized state.
beautified
cast -> call

changed, in order to retrieve the list of the currently loaded plugins.
updated readme