Traio

Among the various added tests, fixes and code cleanup:
- tasks can be retrieved and awaited (they are now futures)
- removed the `AsyncTask.join` method in favor of awaiting the task itself
- No more TaskException: you get directly the exception which popped up
- You can continue adding tasks and awaiting them on a nursery as long as you don't join the nursery

- Adds checks preventing to use as a synchronous context manager
- Better handles Cancellation in various scenarios
- Tests cleanup and split
- Added Travis and Coveralls integration

Supports:

- Creating nurseries
- adding awaitables to a nursery (coroutines or futures)
- setting a timeout on a nursery
- by default, an error in a task will bubble at top level. There is an option to ignore exceptions on particular tasks with the `bubble` parameter of `start_soon`
- by default, a nursery will wait for all its tasks to terminate, except if a task is marked as `master`. A master task termination will cause the the nursery to cancel (with all pending tasks)

This is tested supposed to work on python >= 3.5 (tested up to 3.7).