Support for node 0.11.x

As the title mentions, this pull request is my first attempt to support node v0.11.x.
I mainly used nan.h to give me the macros I needed to support node 0.8, 0.10, 0.11 without much effort.
Just for reference, I was able to build this on v0.8.26, v0.10.22, v0.11.13. Other versions are probably also supported, although nan.h didn't appear to go along with v0.11.12 that well.
I ran npm test (node test/unittest.js) and everything succeeded, but I believe the current test base (test/unittest.js) is kind of thin, and we should have much much more tests.

I have another pending branch at https://github.com/chrisdew/protobuf/pull/42 but for convenience purposes I made a different branch for this one and I would be very happy if you could review the source and hopefully pull it in the main repository.