Cleaning up submit-settings.php, Fixed #56

The most recent problems we've had with Dropplets so far have been with the `submit-settings.php` file. It contains a bit of *meta-PHP*, whereby we're representing data (the settings) as code, and we've written code which writes code.

In order to maintain backwards compatibility (and because I'm not great at PHP, I'm afraid), I've kept it largely how it is. However, I've made it easier to modify and kept the manual PHP writing to a minimum. However as time goes on I recommend this be changed to a hash map ...

``` php
$settings[$key] = $value;
```

... in a future release. Perhaps with some functions adding to check if settings exist etc.