Nice article and useful solution.
On the Drupal best practices side, you could improve one important part:
Drupal uses a couple of standardized ways to include files. inlude_once is never used.
Best best way:
Use the Libaries API contrib module.
The library is placed in the sites/libraries folder. This way, the module and library are not hard linked to each other.
The basic drupal way:
Include the file in your module info file:
files = lib/Pushover/Pushover.php