create vee-eight-4.9 branch

The V8 team just cut the 4.9 branch so I started working on its integration.

That's still a work in progress but I'm creating the PR now because I have issues with a failing test and we will have to discuss about what we do with the V8 APIs that have been deprecated in 4.9 (as an example, I fixed the deprecation warning about String::NewFromOneByte here). The branch cannot be built without manually copying the trace_event dependency (see https://github.com/nodejs/build/issues/304)

cc @ofrobots @nodejs/v8