Expose native on the node-pty module again so that xterm.js can upgrade

[Tyriar]

Tests within xterm.js rely on .native from node-pty v0.4, as a temporary workaround so that xterm.js can move forward I want to expose .native again but not as a public API.

https://github.com/sourcelair/xterm.js/blob/8ece306dbe9c7aee13cce9c60fae74202f8660ee/src/Terminal.integration.ts#L86

@jerch I think some of your finer-grained APIs in #73 will let us remove the dependency on native. If so we should consider pulling those particular APIs in first 😃

[jerch]

@Tyriar Well the RawPty class could be used instead, it is kinda a wrapper around all low level pty stuff on unix.