Cannot use in (offline) tests

In test mode (at least in environments without networking), the `` tag that this addon injects won't run and Stripe won't be defined. That means `Stripe.setPublishableKey` won't exist and the initializer will fail.

The smallest change I can think of that would work (which I've used locally successfully) is

``` js
// app/services/stripe.js
function createToken (card) {
Ember.assert("Cannot create Stripe token in test environment", config.environment !== 'test');
// ...
}

// app/initializers/stripe-service.js
export function initialize() {
if (config.environment === 'test') { return; }
// ...
}
```

This isn't perfect, but it's pretty good. I end up stubbing out `stripe.createToken` whenever I need to actually run that function, with without a standard stubbing library, I can't think of a good way of doing this here.

Read app/services/stripe.js and app/initializers/stripe-service.js, then run the offline or test-mode suite to reproduce the missing Stripe failure. Done means test mode no longer fails during initialization, while attempting token creation reports the intended test-environment error.

Written by the indexing model from the issue text.