Import: Create new entries and update existing rows.

Allows keep db_match_field into insert using a new
argument $keep_match_field of import method.
This argument is false for a compatibility purposes.
When $keep_match_field=true rows without db_match_field must be ignored.
Show how match_field will be used according to importation method.
Merge pull request #90 from jorix/i-addUpdate

Import: Create new entries and update existing rows.