Features / CTools Export support

FYI this is related: #924236: Primary key auto-detection, support for compound primary keys

Here's some initial work:

• eml_list - should work.
• eml_list_roles - exporting works, importing hasn't been completed.
• eml_request - needs work for both exporting & importing.
• eml_subscription - needs work for both exporting & importing.

The last three tables are made more complicated by the fact they use multiple primary keys, which neither CTools nor Features support out of the box. That said, some initial work on this has been done for eml_list_roles - the exporting should work fine, just need to do the importing & other callbacks.

Of course in use having the emf_subscription table exportable can be A Bad Thing(tm) because all of a sudden the Bulk Exporter & the Features Create pages will be trying to list tens and hundreds of thousands records for exporting. This new patch has commented out that part, but left the code still there for others to review.