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 is no a good reason for this. I've store the strings as std::string and when I try the first attemp I probably changed it, because my c ++ was a little rusty, and this was the first thing that worked for me ;) I pushed a change that use strcmp