Add NODE_EXTERN to node::Start

[joelrbrandt]

I'm building node as a DLL on Windows (which works awesome by the way) for embedding inside another application.

node::Start isn't currently exported. It's a useful function.

Any chance we can add this to both v0.6 and master? Thanks!

[bnoordhuis]

I'll be happy to land it in master but not in v0.6, I don't want to change the ABI in stable releases in any way. Can you sign the CLA?

[joelrbrandt]

@bnoordhuis sounds good. I signed the CLA.

I also created #3378 that pulls in to master.

Closing.