Fix for return type of [FMDatabase indexNamesOnTable:]

Thanks for the useful project!

I noticed this method was declared as an `NSArray`

```
/**
 *  Returns the names of all indexes on a given table.
 */
- (NSArray *)indexNamesOnTable:(NSString *)tableName;
```

but implemented as an `NSSet`
 
```
- (NSSet *)indexNamesOnTable:(NSString *)tableName
{
  FMResultSet * schema = [self getSchema];
  NSMutableSet * indexNames = [[NSMutableSet alloc] init];
  ...
  return indexNames;
}
```

I think `NSSet` is the right type.