Various improvement for Fields integration with devel_generate. Now populating Fields attached to all of nodes, users, comments, and terms. Also respecting field settings like max/min and precision/scale for number and text fields.

Support skip-fields in generate-content since some fields like taxonomy can huge allowed values lists and those can memory crash.

#687788 by moshe weitzman. Support file and image fields in D7's devel_generate. Also massively refactor devel_generate into a new directory below devel and lots of little include files for each Field type.