Corrections to simplenews_schema() declarations.

The declarations in simplenews_schema() contain the following errors:

1. The simplenews_mail_spool table incorrectly sets the unsigned attribute on its snid field. The simplenews_subscriber table declares this field without the unsigned attribute; tables which refer to this field in foreign keys should have a compatible declaration.
2. For efficiency, the simplenews_mail_spool should be indexed by snid, tid.
3. All of the foreign keys are declared incorrectly. From the Schema documentation:
   

   'foreign keys': An associative array of foreign keys ('keyname' => specification). Each specification is an array with 'table' and 'columns' elements that form a foreign key for the table.

   'table' is a string specifying the foreign table, and 'columns' is an associative array in the format 'source_column' => 'target_column'.

   Note: Foreign key definitions were added in Drupal 7 for documentation purposes only, and do not modify the database.

   Example from the {node} table:

     'foreign keys' => array(
       'node_revision' => array(
         'table' => 'node_revision',
         'columns' => array('vid' => 'vid'),
       ),
       'node_author' => array(
         'table' => 'users',
         'columns' => array('uid' => 'uid')
       ),
     ),