doc: update BUILDING.md

* General copy-editing

* Move `[sudo] make install` step from general Unix build to own step

* Remove `make test-addons` as that is now covered by `make test`

* Standardize on `/path/to/foo` as used elsewhere in the doc

* Change `node -e` to `./node -e` to avoid using different node binary

PR-URL: https://github.com/nodejs/node/pull/8398


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>