#781088 Reorganize Content Migrate files for efficiency, and also to make it easier to use the individual module code as examples on how to implement the hooks.

#781088 by Island Usurper, loganfsmyth, and mattbatt, fixes to get imagefield and filefield data migrated.

Rewrote content_migrate to migrate one data record at a time, with a drupal_alter() before inserting it into the new table, to allow fields like imagefields to re-organize the data in the new table.

More tweaking of content migrate script, starting to work correctly for at least the cases I have tested so far.

Adding content migrate module. Still needs lots of work, just getting this much committed so it is not lost.