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().

Try to add README.txt to front page via @verbinclude. #fail.

Use existing() for ensuring that the importer in question actually exists in DB or in code. Plus one minor cleanup in switch statement.

Add existing() method that allows for making sure that a configurable actually exists when executing a method on it.
Usage:
try {
  ->existing()->action();
} catch(FeedsNotExistingException ) {}

Fix notices from setConfig() by making sure that all default values are present.