We're currently evaluating your plugin under a local WAMP installation. There's an issue when rendering widgets or shortcodes, the file argument to a require call cannot be found.
This occurs at line 74 of model/widgets/donation_widget.php.
I think the line should be changed from
require(__FILE__ . "/../../../view/donation_form_single.php"); to
require(dirname(__FILE__) . "/../../view/donation_form_single.php");.
This may well be a windows-only issue, I haven't tested under Linux yet.
Otherwise the plugin looks great, thanks!