Add integration tests for success, error and progress

[gr2m]

follow up for #36.

We should add integration tests for the 3 main cases:

1. Create a task which succeeds
2. Create a task which errors
3. Create a task which gets a progress update

I would suggest we mock out @hoodie/store-client using proxyquire for the tests. We can simulate any events we want for the respective cases and can check if the right methods have been called.