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
Same as above - only `Convert` and `Identify`, both supporting sync/async, and don't put "Async" in the function name.