Exposed weak stuff
Expose WeakCallbackInfo through getter.
Updated docs.
Update README.md

Document hidden error message in NanAsyncWorker.
Merge pull request #101 from rvagg/weakstuff

Exposed weak stuff
Merge pull request #102 from rvagg/harmonize

Made NanNull and friends return Locals
Update README.md
Introduce NanCallback::IsEmpty()
Fixes.
Merge pull request #103 from rvagg/isolation

Removed nan_isolate
Merge pull request #105 from rvagg/isempty

Introduce NanCallback::IsEmpty()
Restore nan_isolate and make multiple isolates conditional