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.

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

#701432 pounard, Will White: PHP 5.3: array_shift() expects parameter 1 to be array, null given in (...).

  scheduled tasks than 'import' and 'expire'.

This is a preemptory step to enable cleaner result handling and prepare the ground for Batch API use.