#619374 by moshe weitzman. Better support for filefields even when imagefield is active on other fields.

Implement drush_backend_invoke() for drush imports. This automatically starts a new shell if you are nearing your php memory_limit.

Assure that migrate module is enabled when running updates that depend on it. Yes, disabled but not uninstalled modules run updates too.

#649762 by moshe weitzman. Provide more info in  object, node object, user object, comment object, term object.

Add migrate wipe command which will wipe all nodes of a given content type. We should consider ditching the non batched one thats in the UI. Also removed 'callback' element in favor of magic function naming, as that currently offers invoke hooks ... also avoided one query when upload module is disabled.

use long form comments in install file so php reflection reads them. used by drush in d6 and core in d7. no functionality change.