doc: clarified & split up contribution docs

doc: clarified & split up contribution docs

- clarified the role of "collaborators"
- split out a governance doc
- split out a collaborator guide
- cleaned up the contributing doc
- cleaned up the readme & added collaborators list

PR-URL: https://github.com/iojs/io.js/pull/233
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Mikeal Rogers <mikeal.rogers@gmail.com>
(Note: no explicit "LGTM" from Mikeal)

Discussed at TC meeting 2015-01-07, agreed to push further
amendments to governance to future PRs.