Filefields as their own migrate type (allows multiple files per node)

here is the patch. Note: adding files though the node should still work as before, it's just you can now skip that during node creation and add additional files to the node later (creating a files import map, and enabling the use of the migrate hooks.

@alex_andrascu - No you don't have to use a hook, but note that the import is a different step. First you would import all of your nodes, and then create a new import for the filefield images. When you do the mappings in the settings, you need to know the NEW nid that you want to add each image to. To make that work you have to join a map table to you filefield table.