Fix accidental roll back of previous commit 'Stop PubSubHubbub from subscribing if it is not enabled.'

#711664 neclimdul: guarantee compatibility with CTools 1.4  by declaring that Feeds uses hooks to define plugins via hook_ctools_plugin_plugins().

Add sourceSave() and sourceDelete() methods notifying plugin implementers of a source being saved or deleted. Minor cleanup on sourceForm().

  scheduled tasks than 'import' and 'expire'.

  formalize feed elements.