Fix interleaving of asycnhronous callbacks

[parasyte]

• Create a new context when creating async callbacks; don't just push the current context to the stack.
• This test would work if the callbacks were wrapped with ns.run() or ns.bind(), because this method creates a new context.
• ns.bind() seems to have the same problem, but it has not been tested.
• Added a new test for this specific failure.
• Fixed an existing test which assumed contexts should not be created across async callbacks.
• Fixes Continuations do not work across callbacks unless wrapped with namespace.run() #66

[parasyte]

Won't fix. See #66 (comment)