Creating mutiple data records per import item

In many cases, we import data that needs to be normalized in the Drupal database. Here's the case that motivated this patch: while importing Twitter messages, I wanted to insert both message and user records, which are stored in two separate data tables.

The attached patch achieves this, for Data tables. It provides a new FeedsMultiDataProcessor plugin that behaves like FeedsDataProcessor except for the following differences:

* It allows to select multiple tables that will act as recipients of the import process
* It only shows fields from the selected tables
* It creates a new record in each specified table per imported item - and applies the uniqueness check for each table
* It also allows to check uniqueness per feed_nid (as per other processors) or globally - which is not possible in other processors

Of course, this plugin needs tests but I'd be grateful if some brave souls could try it out first :-) I'm currently using it on a production site.