Overview of changes for 0.2.0
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* Add IrisGMainScheduler for applications that only use Asynchronous tasks
and do not need real threads.
* Allow setting the default scheduler for the process.
* Additional documentation.
* Cleanup of public API.
* Add iris_task_all_of() and iris_task_any_of() to create tasks with
specific task depenedencies.
* Add basic support for OS X.
* Add macros to make working with IrisTask simpler.
* Allow specifying branch control in task handlers using iris_task_add_both().
* iris_arbiter_coordinate() for a message-passing based reader-writer lock.