Check out this thread: How to extend Flowable to add "Business Domain" specific BPMN elements
This post: How to extend Flowable to add "Business Domain" specific BPMN elements lists what I did to implement a custom task.
Edit:
This pull request https://github.com/flowable/flowable-engine/pull/373/files added the http task to flowable and can serve as an example (although it is a bit complex and you will need much less code).