Refactored import process in order to streamline it for every import method (direct, batch and queue)

Imports now go through a Feeds executable class, which is responsible for:

• Switching accounts during each stage of the import;
• Creating batch tasks;
• Running import tasks in the right order (begin, fetch, parse, process and clean);
• Dispatching Feeds events.

A batch object knows on which stage an import is and which operations to run. For some stages, like the parse stage, multiple new batch tasks are created. During the parse stage a bunch of process tasks are created and one 'finish' task. For each import method (direct, batch and queue) a separate batch class exists.

FeedsImportHandler and FeedRefresh are refactored to pass the actual import tasks to the executable.