Use ref/unref terminology instead of addref/release

[jasongin]

Consider renaming:

• napi_reference_addref()
• napi_reference_release()

To:

• napi_reference_ref()
• napi_reference_unref()

To be consistent with the "Ref/Unref" terminology used throughout Node.js.

See nodejs/node#11975 (comment)

[boingoing]

Fixed by #193