Use class constants for valid Respondents.approved values

The -1, 0, and 1 values for 'Respondents.approved' should really be 
changed to class constants. Suggestions:
```
-1 => RespondentsTable::DISMISSED
 0 => RespondentsTable::NOT_ADDRESSED
 1 => RespondentsTable::APPROVED
```

And these integers should be changed to references to those class constants in all of the appropriate getters and setters.