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.