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
Naming these structs `_async` is a bit confusing since their function is more like a context. Consider renaming to e.g. `convert_ctx` or `convert_context` although this isn't critical.