Add WP101_API_KEY

Added WP101_API_KEY for use in wp-config.php. Currently, WP101 allows the key to be hardcoded into the plugin.

```php
// API KEY
// You can hardcode the API key here, and it will be used as a starting value for the key
$_wp101_api_key = '';
```

However, this is lost on a site update (yet retained in the db). I recommend a switch to using a constant that can be added to wp-config, maintaining backwards compatability. I've added the code for you to check out with proper notations in the change log and readme (again pending your approval).