buffer: make additional changes to native API

Address comments and deprecations left in source files. These changes
include:

* Remove the deprecated API.
* Change Buffer::New() that did a copy of the data to Buffer::Copy()
* Change Buffer::Use() to Buffer::New()

PR-URL: https://github.com/nodejs/io.js/pull/1825


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>