Add task API lib.
- Move source mbtiles export to source lib
- Use task API to enforce one-at-a-time export jobs
- Update export template to expect task format
Omit fields that change across platforms in source export fixture.
Remove 'read' stream property from Task object and use only progress stream.
Test that unpipe is called on task.del().
Merge pull request #492 from mapbox/direct-upload-taskapi

Direct upload task API