Proposal for promoting rustbuild to "official status"

[cuviper]

@alexcrichton

• Rust 1.15, starts 2016-11-10, releases 2017-01-02 - makefiles are deprecated
• Rust 1.16, starts 2016-12-22, releases 2017-03-16 - makefiles are deprecated
• Rust 1.17, starts 2017-01-02, released 2017-04-27 - makefiles are removed

I believe 2017-01-02 is supposed to be 2017-02-02 to make 6-week cycles. (Nov 10, Dec 22, Feb 2, Mar 16, Apr 27)