Allow a migration to add/edit to fields() for the CSV source plugin. Now matches sql source. Use this feature in game.inc.

A new migrate_example_baseball migration. Handy for populating your site wih loads of test data. Specifically, results from every MLB game from 2000-2009 including listing all starting batters and paitchers. This migration is innovative in that it adds uses a new CSV source plugin and uses dynamic migration. It depends on features module just becuase I was too lazy to programmatically create all the fields in .install.