Implement progress to NanAsyncWorker.

[brett19]

This builds on NanAsyncWorker by providing a NanAsyncProgressWorker which can dispatch progress information back to the originator.

[brett19]

This should have everything we wanted from the discussion against the previous PR by @kkoopa.

[brett19]

Updated PR with fixes for @kkoopa's comments, as well as a fix to resolve a race condition I discovered.
P.S. Had to add a Destroy() method to handle destruction virtually, as NanAsyncProgressWorker has 'pre-work' necessary before its safe to delete the worker.

[jonathanong]

i'm testing this and it's working pretty well. i don't know much about C++ and stuff but i got it to work with @rvagg's help and it was pretty straight forward.

so... a merge please!!!

[rvagg]

This is slated for a 1.5 (#216) but it does need some docs I believe.

@brett19 do you want to add some docs to the README for this?

@kkoopa what do we need for a 1.5?

[brett19]

I am unfortunately extremely busy at the moment, I'll try to get it done next week.

[kkoopa]

What we need and what we want are two possibly separate things. Fixes and such for broken code or new version incompatibilities are what is strictly needed. That includes #223 and a new thing: v8::V8::ContextDisposedNotification has moved to the current isolate. Additionally, what's already in #216 should also be in the new version.

That should be it. Remaining improvements are probably better off for later versions. Oh yeah, we should test against io.js too.

[jonathanong]

👍