Master node map table

I'm adding a new feature that will basically hook into each time migrate saves a node and map it's ID to one table. normally migrate maps to the map table for a specific content set (which it will continue to do for it's own purposes), but often imports for nodes happen across content sets, one fore each node type. If you have quite a few node types, it is much easier for instance to use this one table for importing things like comments, files, etc. Note: your source ids must be unique across all you node imports to use this.