Add notify_fork.
Update executor_wrapper.
Update to match new executor requirements.
Remove unnecessary executor constructors.
Move traits to executor header.
Use an active invoker when invoking a chain on an explicitly specified executor.
Add versioned namespace.
Update loop_scheduler.
Move traits back to type_traits header.

This reverts commit 08b7acd63ca59dc88ddae1d8519a3dc45696c9df.
Add associated_allocator, associated_executor.
Fix shutdown issue.
Add executor_work<>::owns_work() function.
Add support for packaged_task.
Add timer function overloads (dispatch_at, etc) that take an execution context.
Merge branch 'master' into trading_example