What's the recommended way to include vendor library dependencies in your plugins?
Composer!

There's already a package for Stripe: https://packagist.org/packages/stripe/stripe-php


Install composer.
Add a composer.json file in your plugin's root folder.
Add this to the composer.json file:

{
    "require": {
        "stripe/stripe-php": "1.17.2"
    }
}
Run php composer.phar update from your plugin's root folder.
The library will be installed in a /vendor folder in your plugin.
Add an init() method to your main plugin's class file with this in it:

require CRAFT_PLUGINS_PATH.'/pluginhandle/vendor/autoload.php';


And you're done.

Composer will handle autoloading all of the class files, so you don't need to worry about it.

If you ever want to update the Stripe library to a newer release, grab the version number from the packagist site, update your composer.json file with it and run php composer.phar update again.