identifyAsync and convertAsync.

Initial try for https://github.com/mash/node-imagemagick-native/issues/23 and follow https://github.com/mash/node-imagemagick-native/pull/39.

Include minimal tests in test/test.async.js
There's no reason to assign the parameters to local variables and then assign them to members of `async_data`, we can assign them to the latter directly.