[#681588] by frankcarey: removing migrate_messages and putting privatemsg.migrate.inc into migrate_extras. also changing hooks to use the privatemsg namespace instead of migrate_messages.

by frankcarey: added option for a master node table, so multiple node content sets can share a map table.
by frankcarey: cleaned up the module a little bit, reordering functions

by frankcarey: adding the migrate_messages module to the migrate_extras project. It provides integration with the privatemsg module.  It has been tested as working.

by frankcarey: committing the initial version of the migrate_extras module. It basically adds a new tab on migrate dashboard called 'extras', which is where mostsubmodules should put there settings. It adds 3 tweaks /setting. 1) Use a pre encrypted password.  It will write the password directly to the database without re-encrypting it. Good for use in importing other CMS that already encryt their passwords. 2) Use all user fields. This will make all the core user fields available to the content set, useful for more advanced and drupal-to-drupal migrations. 3) Set UID explicitly. This adds a field to import users keeping the same UID, which can possibly overwrite existing users, so there are cautions about using it, but it can be very useful. This uid part is untested.