Code of Conduct, mk II

Changes from the original:

* does not include contribution policy and governance documents
* explicitly defines the scope of the Node.js Project
* explicitly defines harrassment
* explicitly defines consequences
* adds grievance policy
* introduces notion of specific points of contact for abuse reports; still need a list of actual names to go into the policy
* adds credit and link to the npm, Inc. Code of Conduct
* incorporates community feedback

Please limit discussion on this issue to the Code of Conduct itself:
  * ambiguous or confusing wording
  * typos
  * contradictions
  * scope of the document

Discussion of the necessity or relevance of a Code of Conduct for the Node.js Project is irrelevant to this PR, and should be conducted elsewhere.
To handle and respond to complaints, to make recommendations as to how conduct violations should be handled, and to request that people engaging in unacceptable behavior stop doing so. The Node.js project is small enough that I anticipate there will be overlap between these groups, but I wouldn't assume that everyone who is in a position to respond to conduct issues will necessarily be an administrator, and vice versa.

To make it clearer, the purpose of having an abuse team is to have a broad enough membership that it's likely that there will be at least one member of the team that people experiencing harassment can approach without discomfort.