Define 'async' -> 'asynchronous' alias (restores backwards compatibility)
Define 'async' -> 'asynchronous' alias (restores backwards compatibility)

Following up on 152c7e753ef I'm now programatically adding an
'async' alias that defers to the real 'asynchronous' property.

For more details refer to the following
issues and pull requests on GitHub:

 - https://github.com/xolox/python-executor/issues/9
 - https://github.com/xolox/python-executor/pull/11
 - https://github.com/xolox/python-executor/pull/13

xolox/python-executor