[WIP] V7.0.0 proposal (beta)

This is the start of the v7.0.0 release proposal. Each week until the release a new beta build will be generated from this branch for testing.

Note: this branch includes the beta V8 5.4 commits from https://github.com/nodejs/node/pull/8317. *This PR has not yet landed in master but is expected to do so before v7.0.0 is released. Those commits are included here in order to allow us more time to test v7.0.0 with V8 5.4

This currently does not yet include the release commit that switches to a release build. Nor does it currently include the changedocs. I will generate both of those later.

I had planned on doing a beta build this week but will wait until next week as my schedule is rather busy at the moment.

This is a work in progress PR. Do not land!

/cc @nodejs/ctc @nodejs/citgm @nodejs/testing