v1.1.0 of multitasking

Cooperative multitasking using asynchronous tasks, with support for forced task termination with onExit handlers. Changelog excerpt: - Fixed a bug that did not take into account that in Dart, a function cannot return a Future>result. Dart automatically flattensthis value to Future. - Changed signature of the following methods in the Taskclass: Future> run(), Future start()to Task run(), void start(). First method (run) was changed because Dart blocks Future> Task.run)until the task completes, the second method (start) was changed to be consistent with the first.