non-unique / non-foreign key indexes, case sensitive columns, ignore triggers

Include non-unique, non-foreign-key indexes.  I.e. when you set db_index=True on your column definition.

It isn't uncommon to add "COLLATE utf8_bin" on a mysql column to make it case sensitive in mysql (although Django doesn't make this easy), and prior to these changes this causes the script to emit invalid SQL.  Postgres is case sensitive by default, so its reasonable just to ignore COLLATE utf8_bin.  

The script doesn't cope with triggers, and they look pretty hard to convert automatically.  So update readme to suggest --skip-triggers.  Otherwise triggers cause the script to fail